V4L/DVB (12121): pvrusb2: Improve handling of routing schemes

The pvrusb2 driver has a concept of "routing scheme" which defines
which physical inputs should be connected based on application's
choice of logical input.  The correct "routing scheme" depends on the
specific device since different devices might wire up their muxes

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>