V4L/DVB (7320): pvrusb2: Eliminate timer race during tear-down

The pvrusb2 tear-down logic was clearing two timers before stopping
its internal work queue.  That left a tiny window open where the work
queue might run after the timers are stopped, possibly starting them
again.  This could lead to dangling pointers and an oops.  Solution:
Kill the work queue first, then delete the timers.

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