[media] gspca - sonixj: Fix a zero divide in isoc interrupt

In case of short marker, the number of received packets was not
incremented doing a zero divide when computing the filling rate.

Reported-by: Hans Petter Selasky <hans.petter.selasky@bitfrost.no>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>