media: uapi: videodev2: Fix comment for 12-bit packed Bayer formats

For 12-bit packed Bayer formats: every two consecutive samples are
packed into three bytes. Fix the corresponding comment.

Signed-off-by: Mehdi Djait <mehdi.djait@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>