V4L/DVB (11715): gspca - main: Set the number of packets per ISOC message.

The number of packets per isochronous message may now be set by the subdrivers
(default value 32).

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>