ath9k: validate rx antenna settings

Many chips are not able to deal with non-consecutive rx antenna selections
and respond with calibration errors, reset errors, etc.
When an antenna is selected as a tx antenna, also flag it for rx to avoid
chip issues.

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