V4L/DVB (5642): add comment that VO_MODE is also being set.

usbvision_set_video_format: add comment that VO_MODE is also being set.

Signed-off-by: David Warman <dwarman@davidwarman.net>
Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>