Each hw supports a different iface combinations.
Define the supported combinations in each driver,
and save it in wl->iface_combinations.

Since each driver defines its own combinations now,
it can also define its max supported channels, so
we no longer need to save and set it explicitly
in wlcore.

Update wl18xx interface combinations to allow
multiple APs.
Signed-off-by: NEliad Peller <eliad@wizery.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>