V4L/DVB (7708): pvrusb2-dvb: Fix stuck thread on streaming abort

If the device fails to stream, the feed thread will block forever
waiting for buffers.  But while in this state it was not looking for
an exit condition from the driver DVB interface.  This caused the
thread to jam.  Implement a new stop flag (which will be set
appropriately) to tell the thread to stop.

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