iwlwifi: simplify auth/assoc flow

With mac80211 now giving us station information
(via the sta_state callback) before auth/assoc
we can get rid of tx_sync by adding the station
early for the case of managed interfaces. Keep
AP mode actions the same for now.

As we now get the BSSID early, we can also get
rid of iwl_reprogram_ap_sta().

We can still optimise the number of RXONs we
send to the device, but that can be done later.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>