V4L/DVB (13122): gscpa - stv06xx + ov518: dont discard every other frame

When we call gspca_frame_add, it returns a pointer to the frame passed in,
unless we call it with LAST_PACKET, when it will return a pointer to a
new frame in which to store the frame data for the next frame.

The frame pointer was not updated in stv06xx and ov518.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>