V4L/DVB (13192): gspca - pac_common: redesign function for finding Start Of Frame

The original implementation of pac_find_sof() does not always find
the Start Of Frame (SOF) marker. Replace it with a state machine
based design.

Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>