ath9k_hw: force rx chainmask to 7 on AR9100

Most AR9100 devices already have a chainmask of 7 (three antennas), however
on the ones that don't (rx and tx chainmask set to 5), problems with IQ
mismatch calibration have been observed.
This shows up as tx queue hangs (and subsequent hardware resets) if traffic
is sent during this type of calibration.
Forcing the rx chainmask to 7 fixes the calibration issues with no apparent
negative side effects on throughput and stability.

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