V4L/DVB: gspca_ovfx2: drop first frames in stream if not synced

With the ovfx2 bridge sometimes the first few frames in a stream
would be no good, as the bridge and sensor are not in complete hsync /
vsync yet. This can easily be detected by checking the framesize. So if the
framesize is short and it is one of the 1ste 3 frames after an sd_start,
drop it.

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