V4L/DVB (11174): pvrusb2: Implement reporting of connected sub-devices

The pvrusb2 driver has a function that reports internal state.  It can
be accessed from either the debug interface or as the result of a v4l
log status request.  This change adds information listing sub-devices
to the report.

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