ath9k_hw: tweak noise immunity thresholds for older chipsets

Older chipsets are more sensitive to high PHY error counts, and the
current noise immunity thresholds were based on tests run at QCA with
newer chipsets.

This patch brings back the values from the old ANI implementation for
old chipsets, and it also disables weak signal detection on an earlier
noise immunity level, to improve overall radio stability on affected
devices.

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