[media] dvb-usb: refactor MFE code for individual streaming config per frontend

refactor MFE code to allow for individual streaming configuration
for each frontend

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