[media] v4l: videobuf2: Fix typos in comments

The buffer flags are incorrectly referred to as V4L2_BUF_FLAGS_* instead
of V4L2_BUF_FLAG_* in comments. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>