wifi: mac80211: refactor some key code

There's some pretty close code here, with the exception
of RCU dereference vs. protected dereference. Refactor
this to just return a pointer and then do the deref in
the caller later.

Change-Id: Ide5315e2792da6ac66eaf852293306a3ac71ced9
Signed-off-by: Johannes Berg <johannes.berg@intel.com>