[media] V4L: soc-camera: split a function into two

The soc_camera_power_set() function processes two cases: power on anf off.
These two cases don't share and common code, and the function is always
called with a constant power on / off argument. Splitting this function
into two removes a condition check, reduces indentation levels and makes
the code look cleaner.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>