clk: max77686: Correct callback used for checking clock status

Changing status of clock gates in max77686 requires i2c transfers, which
can sleep, so this is done in prepare and unprepare callbacks. Due to
this, checking whether whether the clock is ungated must be done
in is_prepared() callback as well, for consistency.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>