ath: relax regulatory rules for default regd code

Most AP devices out there do not have a properly programmed regulatory
domain code, which means they are limited to US frequencies by default.

This has been a major annoyance for a long time now. Since no fix for
the manfuacturing process seems to be forthcoming, the only sane option
seems to be to allow users to change the country code in such cases.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45252 3c298f89-4303-0410-b956-a3cf2f4a3e73