V4L/DVB (5507): Pvrusb2: Gather USB bus address info and report it

The V4L2 API requires a unique bus_info string returned as part of the
v4l2_capability structure.  These changes gather up the USB address
information, from the underlying device, into a string and report that
out through v4l2 and via sysfs (for completeness).

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