[media] v4l: Use full 32 bits for buffer flags

The buffer flags field is 32 bits but the defined only used 16. This is
fine, but as more than 16 bits will be used in the very near future, define
them as 32-bit numbers for consistency.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>