ath9k: prevent calibration during off-channel activity

Previously the software scan callback was used to indicate to the hardware,
when it was safe to calibrate. This didn't really work properly, because it
depends on a specific order of software scan callbacks vs. channel changes.
Also, software scans are not the only thing that triggers off-channel
activity, so it's better to use the newly added indication from mac80211 for
this and not use the software scan callback for anything calibration related.

This fixes at least some of the invalid noise floor readings that I've seen
in AP mode on AR9160

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