ath9k: Fix noisefloor history update for extn chains

Before doing hw reset the current channel's noisefloor readings
are updated into history buffer. The extension chain's readings
are considered only if the current channel was configured in HT40.
While moving from HT40 to HT20, the extn chain's readings are
skipped though the current channel is in ht40. This patch updates
extn chain reading based on channel flag.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>