ath10k: support get/set antenna configurations.

Verified that target's tx/rx chain register is set appropriately,
and that the tx rate goes down as number of chains
decrease, but I did not actually try to verify antenna
ceased to transmit when disabled.

kvalo: move ar->supp_*_chainmask initialisation to ath10k_mac_register()

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>