wlcore: wl18xx: allow CCK rates for AP mode

12xx chips allow only OFDM rates in AP mode for BT-Coex purposes. This
is no longer required in 18xx chips, starting with FW 8.6.0.0.8.

Update the min allowed FW version in 18xx to support this functionality.

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>