media: v4l2-flash: Flash ops aren't mandatory

None of the flash operations are mandatory and therefore there should be
no need for the flash ops structure either. Accept NULL.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Reviewed-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>