[media] videodev2.h: Fix V4L2_PIX_FMT_YUV411P description

YUV 4:1:1 uses 12 bits per pixel on average, not 16.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>