mac80211: allow driver access to TKIP RX P1K

When the driver wants to pre-program the TKIP
RX phase 1 key, it needs to be able to obtain
it for the peer's TA. Add API to allow it to
generate it.

The generation uses a dummy on-stack context
since it doesn't know the RX queue.

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