media: ov2685: Assign ret in default case in s_ctrl callback

Assign ret in the default case for s_ctrl callback. This can't happen but
still may result in compiler warnings.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>