mac80211: fix some key comments and code

The key documentation is slightly out of date, fix
that. Also, the list entry in the key struct is no
longer used that way, so list_del_init() isn't
necessary any more there.

Finally, ieee80211_key_link() is no longer invoked
under RCU read lock, but rather with an appropriate
station lock held.

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