wlcore: determine AP extra rates correctly

Don't use the ht_mode module parameter for determining AP supported
rates. We can rely on channel type, since HT40 won't be enabled if our
HT cap doesn't support it.

Enable MIMO only if there enough antennas, and rely on per-peer rate
limitation to prevent IOPs.

Signed-off-by: Arik Nemtsov <arik@wizery.com>