mac80211: fix work race

When we stop an interface, the work on it may still be pending
or running. We do cancel the timer, but we do not currently
protect against the work struct. The race is very unlikely to
hit -- it'll happen only when the driver is using mac80211's
workqueue to run long-running tasks and the sta/mesh works are
delayed for quite a bit.

This patch fixes it by cancelling the work explicitly.

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