V4L/DVB (7713): pvrusb2: Implement cleaner DVB kernel thread shutdown

Earlier fix to handle DVB feed thread aborts was overly-aggressive.
We can take better advantage of what kthread_stop() can do.  This
change simplifies things.

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