if_iwm - Properly implement iwm_wme_update callback function.

if_iwm - Properly implement iwm_wme_update callback function.

* Inspired by iwn(4) and Linux iwlwifi.

* Read wme parameters into a buffer within struct iwm_vap in
  iwm_wme_update().

* If we aren't associated yet, the new settings will soon be sent
  by iwm_mvm_mac_ctxt_changed() during association.

* If we are already associated, explicitly call iwm_mvm_mac_ctxt_changed()
  from iwm_wme_update() to send the new settings to the firmware.

* Change iwm_mvm_ac_to_tx_fifo mapping, to fit the freebsd net80211
  WME stream class numbering, instead of Linux's enum ieee80211_ac_numbers.