ath9k_hw: remove aniState->noiseFloor

I don't know why somebody decided to keep a cached copy of beacon rssi in a
variable called 'noiseFloor', but the caching is unnecessary and the variable
name is confusing, so let's just get rid of it entirely.

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