V4L/DVB (13298): gspca - pac207/pac7311/mr97310a: Simplify pac_find_sof.

Remove struct sd dependency from pac_find_sof() function implementation.
This step prepares separation of pac7302 and pac7311 specific parts of
struct sd.

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>