[media] gspca_sonixb: Rewrite start of frame detection

Our old start of frame detection code wrongly assumes that the sof
marker always lives at the beginning of the frame. At least for the
0c45:602a camera this is not the case. This patch also improves
the framerate from 28 fps to 30 fps with the 0c45:6005 and 0c45:6007

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