[media] uvcvideo: Fix bytesperline calculation for planar YUV

The formula used to calculate bytesperline only works for packed format.
So far, all planar format we support have their bytesperline equal to
the image width (stride of the Y plane or a line of Y for M420).

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>