mac80211: cancel work instead of waiting for it to do nothing

If the sdata work is pending while the interface is stopped,
we currently flush it. If it's not running this means waiting
for it to run, which could take a while if the workqueue is
backlogged. However, the work exits right away if it starts
to run while the interface is already stopping. There's no
point in waiting for that, so use cancel_work_sync() instead.

Reported-by: Ben Greear <greearb@candelatech.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>