ath9k_hw: set ANI firstep as absolute values instead of relative

On older chips, the INI value differ in similar ways as cycpwr_thr1, so
convert it to absolute values as well.

Since the ANI algorithm is different here compared to the old
implementation (fewer steps, controlled at a different point in time),
it makes sense to use values similar to what would be applied for newer
chips, just without relying on INI defaults.

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