[media] v4l: Correct create_bufs documentation

Patch id 6016af82 ("[media] v4l2: use __u32
rather than enums in ioctl() structs") unintentionally changes the type of
the format field in struct v4l2_create_buffers from struct v4l2_format to
__u32. Revert that change.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>