ath9k_hw: clean up ANI OFDM trigger handling

Adjust ah->config.ofdm_trig_{high,low} when setting noise immunity values
to simplify threshold checks in ath9k_hw_ani_monitor

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