V4L/DVB (7687): pvrusb2: Fix oops in pvrusb2-dvb

The pvrusb2-dvb feed thread cannot be allowed to exit by itself
without first waiting for kthread_should_stop() to return true.
Otherwise the driver will have a dangling task_struct context, which
will cause a very nasty kernel oops.

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