V4L/DVB (4746): HM12 is YUV 4:2:0, not YUV 4:1:1

Fix comment in videodev2.h

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>