wl12xx: replace wl->mac_addr with vif->addr

The mac address of the interface already exists in vif->addr.
Use it instead of wl->mac_addr.

It seems that due to some fw bug, we still need to set nvs->mac
to the actual mac addresss, otherwise the fw doesn't function
well (e.g. can't get dhcp address).
Thus, use wl->mac_addr for this purpose, and don't delete it yet.

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