wlcore/wl12xx/wl18xx: configure max_stations per-hw

Each hw supports a different max stations (connected to the
same ap). add a new wl->max_ap_stations and use it instead
of the current common AP_MAX_STATIONS.

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