ath9k_hw: reduce ANI firstep range for older chips

Use 0-8 instead of 0-16, which is closer to the old implementation.
Also drop the overwrite of the firstep_low parameter to improve
stability.

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