[media] gspca: reset image_len to 0 on LAST_PACKET when discarding frame

Reset image and image_len to NULL/0 on LAST_PACKET when we're in discard
frame mode, just like we do when not discarding the current frame.

The new se401 driver uses image_len for SOF/EOF detection and thus depends on
this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>