[media] dvb_usb_v2: merge get_ts_config() to get_usb_stream_config()

Piggypag TS type callback to USB stream callback and change
callback name slightly to fit better.
Both of those are rather rare callback and has a relation. Transport
Stream, TS, is input stream and USB stream is output stream of
DVB USB bridge.

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