ath9k_hw: do not lower ANI setting below default on AR913x

When the amount of noise fluctuates strongly, low immunity settings
can sometimes disrupt signal detection on AR913x chips. When that
happens, no OFDM/CCK errors are reported anymore, and ANI tunes the
radio to the lowest immunity settings.
Usually rx/tx fails as well in that case.

To fix this, keep noise immunity settings at or above ANI default level,
which will keep radio parameters at or above INI values.

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