ath9k: Read noise floor calibration data from eeprom

AR9003 devices can have calibrated noise floor values
which can be used instead of hard coded one. Read them
from eeprom and save interpolated value in nf limits for
the current channel.

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