media: i2c: ov5645: Refactor ov5645_set_power_off()

Factor out all the power off logic, except clk_disable_unprepare(), to a
new function __ov5645_set_power_off().

This allows ov5645_set_power_on() to excplicitly clean-out the clock
during the error-path.

The following smatch warning is fixed:
drivers/media/i2c/ov5645.c:690 ov5645_set_power_on() warn: 'ov5645->xclk' from clk_prepare_enable() not released on lines: 690.

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>