[media] gspca_xirlink_cit: Frames have a 4 byte footer

At least on the ibm netcam pro frames have a 4 byte footer, take this
into account when calculating sizeimage.

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