media: ov7670: don't access registers when the device is powered off

Since commit 3d6a8fe2 ("media: ov7670: hook s_power onto v4l2 core"),
the device is actually powered off while the video stream is stopped.

So now set_format and s_frame_interval could be called while the device
is powered off, but these callbacks try to change the register settings
at this time.

The frame format and framerate will be restored right after power-up, so
we can just postpone applying these changes at these callbacks if the
device is not powered up.

Fixes: 3d6a8fe2 ("media: ov7670: hook s_power onto v4l2 core")

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Tested-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>