ath9k_hw: abstract loading noisefloor

This is the last call on calib.c which acceses PHY stuff,
with this change we calib.c is now generic between both
all supported hardware families.

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