[media] v4l: Rename V4L2_MBUS_FMT_GREY8_1X8 to V4L2_MBUS_FMT_Y8_1X8

For consistency with the V4L2_MBUS_FMT_Y10_1X10 format.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>