mac80211: fix key hwaccel race

The previous key locking patch left a small race: it would be possible
to add a key and take the interface down before the key todo is run so
that hwaccel for that key is enabled on an interface that is down. Avoid
this by running the todo list when an interface is brought up or down.

This patch also fixes a small bug: before this change, a few functions
used the key list without the lock that protects it.

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