ath9k: Fix invalid noisefloor reading due to channel update

While switching b/w HT20/40, the current channel's nf values
are updated into history buffer. Since the current channel's
channel type, channel flag got updated before reading
nf value from hw. This channel type mismatch is causing invalid
readings when hw is on ht20 but getnf tries to read on extn chains.

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