V4L/DVB (13117): gspca - pac7311: remove magic value for SKIP.

Change the magic value 0xaa to SKIP for better understandability.

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>