V4L/DVB (13299): gspca - pac7311: Extract pac_start_frame.

Creating the start of the frame is done in the same way for pac7302
and for pac7311. Extract this common part to the pac_start_frame()
function.

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>