V4L/DVB (13925): gspca - main: Change the check of the USB video interface.

Some webcams have many interfaces with the same interface class, so the
previous interface check did not work.
The new code checks if the interface number is zero or the only one.

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