ARM: SAMSUNG: fix clk_enable() WARNing in S3C24XX ADC

Convert clk_enable/clk_disable to clk_prepare_enable/clk_disable_unprepare
calls as required by common clock framework. Removes this warning on probe:

 ------------[ cut here ]------------
 WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:889 __clk_enable+0x28/0x9c()
 Modules linked in:
 CPU: 0 PID: 1 Comm: swapper Not tainted 3.19.0+ #46
 [<c0013a98>] (unwind_backtrace) from [<c0010d8c>] (show_stack+0x10/0x14)
 [<c0010d8c>] (show_stack) from [<c001b018>] (warn_slowpath_common+0x7c/0xa4)
 [<c001b018>] (warn_slowpath_common) from [<c001b0d0>] (warn_slowpath_null+0x18/0x20)
 [<c001b0d0>] (warn_slowpath_null) from [<c01a5f80>] (__clk_enable+0x28/0x9c)
 [<c01a5f80>] (__clk_enable) from [<c01a600c>] (clk_enable+0x18/0x2c)
 [<c01a600c>] (clk_enable) from [<c001860c>] (s3c_adc_probe+0x11c/0x18c)
 [<c001860c>] (s3c_adc_probe) from [<c0153a10>] (platform_drv_probe+0x30/0x78)
 [<c0153a10>] (platform_drv_probe) from [<c01523e0>] (driver_probe_device+0xb0/0x1fc)
 [<c01523e0>] (driver_probe_device) from [<c01525dc>] (__driver_attach+0x68/0x88)
 [<c01525dc>] (__driver_attach) from [<c0150df0>] (bus_for_each_dev+0x70/0x94)
 [<c0150df0>] (bus_for_each_dev) from [<c0151c80>] (bus_add_driver+0xdc/0x1c4)
 [<c0151c80>] (bus_add_driver) from [<c0152ba4>] (driver_register+0x9c/0xe0)
 [<c0152ba4>] (driver_register) from [<c03041cc>] (adc_init+0x10/0x34)
 [<c03041cc>] (adc_init) from [<c00087fc>] (do_one_initcall+0x110/0x1cc)
 [<c00087fc>] (do_one_initcall) from [<c02ffccc>] (kernel_init_freeable+0xf4/0x1ac)
 [<c02ffccc>] (kernel_init_freeable) from [<c022f4fc>] (kernel_init+0x8/0xe0)
 [<c022f4fc>] (kernel_init) from [<c000e098>] (ret_from_fork+0x14/0x3c)
 ---[ end trace f4a1ea39a114fecf ]---

Signed-off-by: Sergiy Kibrik <sakib@meta.ua>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>