V4L/DVB (13564): gspca - main: Implement vidioc_enum_frameintervals.

Some drivers support multiple frameintervals (framerates), make gspca able to
enumerate them.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>