media: ov7670: split register setting from set_fmt() logic

This will allow us to restore the last set format after the device returns
from a power off.

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>