[media] bttv: there may be multiple tvaudio/tda7432 devices

Probe for additional tvaudio devices, and allow tvaudio+tda7432 to
co-exist. My STB TV PCI FM bttv card has a tda7432, a tda9850 and a
tea6420 and with this patch it finally works again (probably for the
first time in many years).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>