V4L/DVB (7311): pvrusb2: Allow digital streaming without encoder firmware

The encoder is not a part of the pipeline when in digital mode, so
streaming is OK in this case even when the encoder's firmware is not
loaded.  Modify the driver core handling of this scenario to permit
streaming.

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