clocksource/drivers/sh_mtu2: Fix multiple shutdown call issue

On the r7s72100 Genmai board the MTU2 driver currently triggers a common
clock framework WARN_ON(enable_count) when disabling the clock due to
the MTU2 driver after recent callback rework may call ->set_state_shutdown()
multiple times. A similar issue was spotted for the TMU driver and fixed in:
452b1324 clocksource/drivers/sh_tmu: Fix traceback spotted in -next

On r7s72100 Genmai v4.3-rc7 built with shmobile_defconfig spits out the
following during boot:

sh_mtu2 fcff0000.timer: ch0: used for clock events
------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:675 clk_core_disable+0x2c/0x6c()
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.3.0-rc7 #1
Hardware name: Generic R7S72100 (Flattened Device Tree)
Backtrace:
[<c00133d4>] (dump_backtrace) from [<c0013570>] (show_stack+0x18/0x1c)
[<c0013558>] (show_stack) from [<c01c7aac>] (dump_stack+0x74/0x90)
[<c01c7a38>] (dump_stack) from [<c00272fc>] (warn_slowpath_common+0x88/0xb4)
[<c0027274>] (warn_slowpath_common) from [<c0027400>] (warn_slowpath_null+0x24/0x2c)
[<c00273dc>] (warn_slowpath_null) from [<c03a9320>] (clk_core_disable+0x2c/0x6c)
[<c03a92f4>] (clk_core_disable) from [<c03aa0a0>] (clk_disable+0x40/0x4c)
[<c03aa060>] (clk_disable) from [<c0395d2c>] (sh_mtu2_disable+0x24/0x50)
[<c0395d08>] (sh_mtu2_disable) from [<c0395d6c>] (sh_mtu2_clock_event_shutdown+0x14/0x1c)
[<c0395d58>] (sh_mtu2_clock_event_shutdown) from [<c007d7d0>] (clockevents_switch_state+0xc8/0x114)
[<c007d708>] (clockevents_switch_state) from [<c007d834>] (clockevents_shutdown+0x18/0x28)
[<c007d81c>] (clockevents_shutdown) from [<c007dd58>] (clockevents_exchange_device+0x70/0x78)
[<c007dce8>] (clockevents_exchange_device) from [<c007e578>] (tick_check_new_device+0x88/0xe0)
[<c007e4f0>] (tick_check_new_device) from [<c007daf0>] (clockevents_register_device+0xac/0x120)
[<c007da44>] (clockevents_register_device) from [<c0395be8>] (sh_mtu2_probe+0x230/0x350)
[<c03959b8>] (sh_mtu2_probe) from [<c028b6f0>] (platform_drv_probe+0x50/0x98)

Reported-by: Chris Brandt <chris.brandt@renesas.com>
Fixes: 19a9ffb3 ("clockevents/drivers/sh_mtu2: Migrate to new 'set-state' interface")
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>