ath9k_hw: Disable Walsh spatial spreading for 2 chains

The Walsh bit is disabled for regulatory consideration.

FCC limit for walsh enable is lower than that for walsh disable. So
disabling walsh bit will not limit tx power/affect tx power even in
cases where we are not FCC limited (most client cards). If the tx
power is not FCC limited, then enabling/disabling walsh bit will
not affect Avg. EVM/overall performance in any visible manner.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>