ath9k_hw: simplify noisefloor calibration chainmask calculation

The noisefloor array index always corresponds to the rx chain number it
belongs to (with an offset of 3 for the extension chain).

It's much simpler (and actually more correct) to directly use the
chainmask to calculate the bitmask for the noisefloor array, instead of
using these weird chip revision checks and hardcoded mask values.

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