ath9k_hw: fix invalid extension channel noisefloor readings in HT20

When the hardware is configured in HT20 mode, noise floor readings for
the extension channel often return invalid values, which keep the
values in the NF history buffer at the hardware-specific maximum limit.
Fix this by discarding the extension channel values when in HT20 mode.

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