V4L/DVB (8971): initial driver for af9013 demodulator

- initial driver for the Afatech AF9013 demodulator

Signed-off-by: Antti Palosaari <crope@iki.fi>
[mchehab.redhat.com: having a global var called 'debug' is not a good idea. rename it to af9013_debug]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>