V4L/DVB (13897): gspca - main: Check the interface class at probe time.

The USB video interface was checked as having the number zero, but
some webcams have other values. The test is now done on the interface
class which may be either 255 (vendor spec) or 0 (class per interface).

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