[media] s2255drv: fix payload size for JPG, MJPEG

length is the size of the buffer, not the payload. That's set using
vb2_set_plane_payload().

Signed-off-by: Dean Anderson <linux-dev@sensoray.com>
Cc: <stable@vger.kernel.org>      # for v3.15 and up
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>