ath9k: implement .get_antenna and .set_antenna

On MIMO chips this can be used to enable/disable hardware chains, ensuring
that the MCS information is updated accordingly.
On non-MIMO chips with rx diversity (e.g. 9285), this configures the rx
input antenna.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>