media: ov7670: hook s_power onto v4l2 core

The commit 71862f63 ("media: ov7670: Add the ov7670_s_power function")
added a power control routing. However, it was not good enough to use as
a s_power() callback: it merely flipped on the power GPIOs without
restoring the register settings.

Fix this now and register an actual power callback.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>