V4L/DVB (7723): pvrusb2: Clean up input selection list generation in V4L interface

Change how list of possible pvrusb2 inputs is generated to include
only those interfaces that make sense for the interface instance.

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