mac80211: fix another key non-race

The code here is only not racy because all the
places that assign the pointers it uses are
holding the sta_mtx as well as the key_mtx and
so can't race against this because this code
holds the sta_mtx. But that's not intuitive,
so fix it to hold the key_mtx.

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