wl12xx: check bss_conf->assoc on CHANGED_BSSID

with the new auth/assoc redesign, we get CHANGED_BSSID
indication before CHANGED_ASSOC indication, while our
CHANGED_BSSID handling block assumes we are already
associated.

Fix it by checking we are either in ibss mode, or
already associated.

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