V4L/DVB (10237): pvrusb2: Generate a device-unique identifier

Implement a new internal function to create a string device
identifier.  This ID stays with the specific device, making it useful
to user space to identify specific devices.  We use the serial number
if available; otherwise we give up and just spit out a unit/instance ID.

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