staging: wfx: change the way the station associate to an AP

Chipset need two steps to associate with an AP:
   1. it start receiving beacon from the AP (done with wfx_do_join())
   2. it sent the association request (done with wfx_join_finalize())

The join request (see hif_join()) contains basic rates, beacon interval
and bssid to connect, so we trig on these events for the first step.

The second step is obviously associated to the event BSS_CHANGED_ASSOC.

Note that conf_mutex is now easier to manage. It is held by
wfx_bss_info_changed() and inner functions does not need to lock it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200410133239.438347-7-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>