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>