mac80211: clean up ieee80211_quiesce

It's a bit odd that there's a return value that only
depends on the iftype, move that logic out of the
function into the only caller that needs it.

Also, since the quiescing could stop timers that
trigger the sdata work, move the sdata work cancel
into the function and after the actual quiesce.

Finally, there's no need to call it on interfaces
that are down, so don't.

Change-Id: I1632d46d21ba3558ea713d035184f1939905f2f1
Signed-off-by: Johannes Berg <johannes.berg@intel.com>