ath9k: Use calibrated noise floor value when available

AR9003 series allows to calibrate noise floor for different frequency
bins. Once it's done it's possible to get more accurate rssi/signal
values over whole frequency band at a given temperature.
The RSSI/signal accuracy reported by calibrated RF cards improves
from 6 to up to 2dB.

This could be interesting for application which require good signal
accuracy like roaming or mesh protocols.

Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>