V4L/DVB (5037): Pvrusb2: Implement multiple minor device number handling

This is the first patch in preparation of the V4L2/IVTV radio interface.
It does away with the assumption of only one minor per device. It also
adds a file to show the radio minor as well. This can be useful for a
program like pvr-radio.c (when it grows up), since this way it can search
for the minor of the /dev/radioX device it opened and use the video minor
of the same driver instance to get to the actual stream.

The implementation looks kinda ugly. Feel free to improve (that is the
reason behind separate patches anyway).

Signed-off-by: Pantelis Koukousoulas <pakt223@freemail.gr>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>