wl12xx: don't join upon disassociation

wl12xx "rejoins" upon every BSS_CHANGED_BSSID notification.
However, there is no need to rejoin after disassociation, so just
filter out the case when the new bssid is 00:00:00:00:00:00.

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