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+0x88/0...