V4L/DVB (10238): pvrusb2: Change sysfs serial number handling

Use the new pvrusb2 internal API to grab the device identifier, rather
than generating it directly.  This unifies some code and make possible
use of that identifier in places other than sysfs.

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