ath: use CTL region from cfg80211 if unset in EEPROM

Many AP devices do not have the proper regulatory domain programmed in
EEPROM. Instead they expect the software to set the appropriate region.
For these devices, the country code defaults to US, and the driver uses
the US CTL tables as well.
On devices bought in Europe this can lead to tx power being set too high
on the band edges, even if the cfg80211 regdomain is set correctly.
Fix this issue by taking into account the DFS region, but only when the
EEPROM regdomain is set to default.

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