media: ar0521: Refactor ar0521_power_off()

Factor out all the power off logic, except the clk_disable_unprepare(),
to a new function __ar0521_power_off().

This allows ar0521_power_on() to explicitly clean-out the clock during
the error-path.

The following smatch warning is fixed:
drivers/media/i2c/ar0521.c:912 ar0521_power_on() warn: 'sensor->extclk' from clk_prepare_enable() not released on lines: 912.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>