[media] v4l: videobuf2: Typo fix

vb2_get_plane_payload() gets the bytesused field for a plane, it doesn't
set it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>