V4L/DVB (13183): gspca: add stv0680 subdriver

This patch adds a new subdriver to gspca supporting cams with the stv0680
bridge (replacing the old in kernel v4l1 driver).

Many thanks to Hans Verkuil for providing me with one of the 2 cams used in
testing this new sub driver.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>