ath9k_hw: allow fast channel change when only CHANNEL_HT changes

The CHANNEL_HT flag is insignificant for fast channel change conditions,
since it does not affect any important part of the hardware reset /
channel setup.
Scanning usually runs with HT disabled, so this change will slightly
improve scan time on many chipsets.

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