V4L/DVB (5094): Pvrusb2: Introduce fake audio input selection

This should allow mplayer pvr:// to start. The trick is that no matter
what actual input we use under this "fake" one, it will be able to do
stereo :-)

Signed-off-by: Pantelis Koukousoulas <pakt223@freemail.gr>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>