[media] em28xx: Multi Frontend (MFE) support

Register multiple FEs for same adapter. After that it is
possible to register two FEs for same adapter. For example
one for DVB-T and one for DVB-C.

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