[iwm] valid_{tx,rx}_ant from radio_cfg is only needed for 8000 family.

• The "if (!data->valid_tx_ant || !data->valid_rx_ant) {" check was getting triggered with a 3165 chipset.

Submitted by: Imre Vadasz <imre@vdsz.com>
Obtained from: DragonflyBSD 3655dfb6fc311fc83e5ce8370dd91b4cd4a37991