media: ov2680: Add an __ov2680_get_pad_format() helper function

Add an __ov2680_get_pad_format() helper function.

This is a preparation patch for adding selections support.

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>