wl18xx: change the number of WLAN addrs per chip

Each 18xx chip contains only 2 real MAC addresses
usable for WLAN, forcing us to use the LAA bit
approach to obtain a third MAC address.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>