V4L/DVB (7297): pvrusb2: Dynamically control range of input selections

This follows from defining the available inputs as device attributes.
This change causes the driver to adjust its list of inputs based on
those attributes.  Now, for example, the FM radio will appear as a
choice only if the hardware supports an FM radio.

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