mac80211: cancel the connection monitor timers/work

In "mac80211: monitor the connection" I forgot to
add code to cancel the new timers & work when the
interface is brought down, which isn't a problem
if you just bring it down, but _is_ a problem when
you destroy the interface. Correct this lapse.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>