[media] go7007: correct a header check: MPEG4 has a different GOP code

Actually it's called "Group of VOP" start code in MPEG4 terminology.
Taken from the 0.9.8-5 version of the out-of-tree wis-go7007 driver.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>