ath9k_hw: do not load noise floor readings when it is running

Noise floor calibration is performed on longcal interval and
the reading will be updated in history buffer. On rare occasions,
the previous noisefloor calibration might not be completed within
the period and trying to load nf reading will be failed. In such
situation, postpone the nf cabliration to next cycle to give
enough time to complete the calibration. This was already taken
care for ar9002 chips.

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