ath9k: add support for the HSR tuner of the Ubiquiti UAP Outdoor+

Without setting the HSR to the selected channel, the WLAN of the UAP
Outdoor+ will exhibit high packet loss in RX.

Based-on-patch-by: Stefan Rompf <stefan@loplof.de>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>