ath9k_hw: fix a noise floor calibration related race condition

On AR5008-AR9002, other forms of calibration must not be started while
the noise floor calibration is running, as this can create invalid
readings which were sometimes not even recoverable by any further
calibration attempts.

This patch also ensures that the result of noise floor measurements
are processed faster and also allows the result of the initial
calibration on reset to make it into the NF history buffer

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