V4L/DVB (12122): pvrusb2: De-obfuscate code which handles routing schemes

This change does not change any outward behavior; it merely chops down
some large if-conditions with embedded assignments into something a
little more maintainable for others (I of course never had a problem
with this...).

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