iio: adc: vf610_adc: Fix division by zero error

In case the fsl,adck-max-frequency property is not present in
the device tree, a division by zero error results during the
probe call on kernel boot (see below). This patch fixes it and
also restores device tree compatibility in case kernels are
booting with old device trees without this property specified.

[    1.063229] Division by zero in kernel.
[    1.067152] CPU: 0 PID: 1 Comm: swapper Not tainted
4.3.0-rc5-00212-gcc88cef #37
[    1.074650] Hardware name: Freescale Vybrid VF5xx/VF6xx (Device Tree)
[    1.081135] Backtrace:
[    1.083694] [<800134a4>] (dump_backtrace) from [<8001369c>]
(show_stack+0x18/0x1c)
[    1.091340]  r7:00000008 r6:8e0ae210 r5:00000000 r4:8e299800
[    1.097146] [<80013684>] (show_stack) from [<80297b1c>]
(dump_stack+0x24/0x28)
[    1.104483] [<80297af8>] (dump_stack) from [<80013608>]
(__div0+0x1c/0x20)
[    1.111421] [<800135ec>] (__div0) from [<802968b4>] (Ldiv0+0x8/0x10)
[    1.117865] [<80424350>] (vf610_adc_probe) from [<803153b4>]
(platform_drv_probe+0x4c/0xac)
[    1.126311]  r10:00000000 r9:8076a5ec r8:00000000 r7:fffffdfb
r6:807cc67c r5:8e0ae210
[    1.134319]  r4:807f6c54
[    1.136915] [<80315368>] (platform_drv_probe) from [<803138bc>]
(driver_probe_device+0x20c/0x2f8)
[    1.145882]  r7:807cc67c r6:00000000 r5:8e0ae210 r4:807f6c54
[    1.151657] [<803136b0>] (driver_probe_device) from [<80313a3c>]
(__driver_attach+0x94/0x98)
[    1.160190]  r9:8076a5ec r8:00000098 r7:00000000 r6:8e0ae244
r5:807cc67c r4:8e0ae210
[    1.168112] [<803139a8>] (__driver_attach) from [<80311cb8>]
(bus_for_each_dev+0x70/0xa4)
[    1.176383]  r7:00000000 r6:803139a8 r5:807cc67c r4:00000000
[    1.182159] [<80311c48>] (bus_for_each_dev) from [<80313318>]
(driver_attach+0x24/0x28)
[    1.190260]  r6:807bb568 r5:8e2a5b00 r4:807cc67c
[    1.194996] [<803132f4>] (driver_attach) from [<80312f50>]
(bus_add_driver+0x1a4/0x21c)
[    1.203113] [<80312dac>] (bus_add_driver) from [<803142a8>]
(driver_register+0x80/0x100)
[    1.211275]  r7:8e2a7dc0 r6:807a8160 r5:80789e14 r4:807cc67c
[    1.217075] [<80314228>] (driver_register) from [<803152f8>]
(__platform_driver_register+0x5c/0x64)
[    1.226216]  r5:80789e14 r4:807a8160
[    1.229877] [<8031529c>] (__platform_driver_register) from
[<80789e30>] (vf610_adc_driver_init+0x1c/0x20)
[    1.239556] [<80789e14>] (vf610_adc_driver_init) from [<800095f8>]
(do_one_initcall+0x94/0x1dc)
[    1.248365] [<80009564>] (do_one_initcall) from [<8076ae34>]
(kernel_init_freeable+0x13c/0x1e0)
[    1.257155]  r10:80794830 r9:8076a5ec r8:00000098 r7:807d5780
r6:807d5780 r5:00000006
[    1.265153]  r4:807a0ee8
[    1.267753] [<8076acf8>] (kernel_init_freeable) from [<80590ef0>]
(kernel_init+0x18/0xf0)
[    1.276021]  r10:00000000 r9:00000000 r8:00000000 r7:00000000
r6:00000000 r5:80590ed8
[    1.284015]  r4:807d5780
[    1.286615] [<80590ed8>] (kernel_init) from [<8000f878>]
(ret_from_fork+0x14/0x3c)
[    1.294278]  r5:80590ed8 r4:00000000

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Acked-by: Fugang Duan <B38611@freescale.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>