ath9k_hw: calculate a much better approximation of channel noise

Currently ath9k presents the internal calibrated noise floor as channel
noise measurement, however this results in highly chip specific values
that are only useful as relative measurements but do not resemble any
real channel noise values.

In order to give a much better approximation of the real channel noise,
add the difference between the measured noise floor and the nominal
chip specific noise floor to the default minimum channel noise value,
which is currently used to calculate the signal strength from the RSSI
value. This may not be 100% accurate, but it's much better than what's
there before.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>