V4L/DVB (7688): pvrusb2: Clean up dvb streaming start/stop

Eliminate the need for a separate pvr2_dvb_fh; since in the DVB
context there can only ever be a single instance then there is no need
for a separate instance to handle streaming state.  This simplifies
the module.  Also move streaming start/stop out of the feed thread and
into the driver's main context - which makes it possible for streaming
start up failures to be detected by the DVB core.

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