V4L/DVB (7296): pvrusb2: Define device attributes for all input modes

Different devices support different input types.  Up until now we've
really been assuming that everyone has an analog tuner, an FM radio,
composite, and s-video inputs.  But as we add other devices, these
assumptions are no longer true.  The way to deal with this is to
define the available inputs as additional device attributes, so that
the driver can adjust its internal behavior accordingly.

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