V4L/DVB (11711): gspca - main: Fix a crash when no bandwidth available

When the bandwidth is not wide enough, the transfer endpoint may be set to
the one of the alternate setting 0. This one may be null and this causes a
divide by 0 oops.

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