V4L/DVB (13118): gspca - pac7311: remove magic values for END_OF_SEQUENCE and LOAD_PAGE.

Change the magic values 0, 254 and 255 to END_OF_SEQUENCE, LOAD_PAGE4 and
LOAD_PAGE3 respectively for better source code readability.

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>