ath9k: merge noisefloor load implementations

AR5008+ and AR9003 currently use two separate implementations of the
ath9k_hw_loadnf function. There are three main differences:

 - PHY registers for AR9003 are different
 - AR9003 always uses 3 chains, earlier versions are more selective
 - The AR9003 variant contains a fix for NF load timeouts

This patch merges the two implementations into one, storing the
register array in the ath_hw struct. The fix for NF load timeouts is
not just relevant for AR9003, but also important for earlier hardware,
so it's better to just keep one common implementation.

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