Make our old BSSID available to iwx_newstate_task() when roaming.

Make our old BSSID available to iwx_newstate_task() when roaming.

ic_bss->ni_bssid has already been overwritten once we enter
iwx_newstate_task() to perform the state transitions necessary
for roaming to our new access point (RUN->AUTH->ASSOC->RUN).

We do however use the BSSID in commands sent to firmware.
Cache our BSSID in struct iwx_node such that firmware commands
keep using the old BSSID while we are still tearing things down.
Switch to the new BSSID only once we start back up in iwx_auth().
This should be consistent from the firmware's point of view.

Same fix as committed for iwm(4) recently.