wl12xx: configure the correct beacon_interval

We didn't update the beacon interval on association
(or on a change notification when working as sta),
so the default interval (100ms) was always used.

Update the beacon interval according to the bss_conf
before starting the sta role (on association).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>