Noise floor is a measurement of the ambient RF noise as seen by your wifi interface. It is not configurable, it is a read only value. It is however used in conjunction with signal strength to calculate the SNR value.
sorry but I dont agree, with N cards you could set the noise floor manually to make the card less vulnerable to the noise. I am using this in the city for some short links, lowering the automatic noise by 5dB to keep my links running stable with 100% CCQ. It really helps because the automatic noise floor is sometimes just too high (-110dB in center of the city)...
I am also missing this feature on the AC products, already posted here a week ago but no answer from support. There is still many features not finished in the AC driver like current tx-power readings etc. I guess we need to wait.