[media] dvb_usb_v2: add .get_ts_config() callback

Add new callback in order to resolve used TS (MPEG transport stream)
configuration. Normal TS is 188 byte payload only but there is
currently 204 byte TS and raw TS supported too.

Traditionally TS type was mapped as a frontend property but it
does not work no longer after we changed from MFE (multi-frontend)
to SFE (single-frontend). So it is not possible to map TS for the
given FE as there could be only one FE instead of multiple.

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