ath9k: add noise floor override option

Introduce a debugfs option to manually override the noise floor,
ignoring the automatically tuned noise floor of the driver/hw.

In my tests with a AR9580 based module and a tx99 5 MHz interferer,
I could tune the noisefloor to -95 dBm or above to allow communication
again. The automatic noise floor calibration sometimes could adapt to
the situation as well, but not reliably and permanently.

I would consider this "feature" experimental and interesting for people
debugging the noise floor calibration or other effects of the hardware.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fit.fraunhofer.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>