ath9k: load noise floor from history after the full chip reset

Currently during the full reset, the nf calibration is always
restarted from the defaults. The noise floor history buffers are
never be used again after the scan and ath reset. This patch
ensures that nf histories are always be used that helps to
improve the signal quality on congested environment

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