V4L/DVB: pvrusb2: New feature to mark specific hardware support as experimental

This adds a flag in the device attribute structure which can be used
to mark support for a particular device as experimental.  Any devices
flagged in this way, when encountered at run-time, will generate a
warning message to the kernel log.

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