[media] af9033: implement get_frontend

Implement the get_frontend function.
The code is derived from the old af9033 driver by Antti Palosaari.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>