V4L/DVB (7303): pvrusb2: Ensure that default input selection is actually valid

Previously the pvrusb2 driver just started with the default input to
be "television".  But if the device doesn't support an analog tuner
then this default must be different.  New logic here selects a
reasonable default based on the actual valid set of available inputs.

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