ath: Add regulatory mapping for Serbia

ath: Add regulatory mapping for Serbia

The country code is used by the ath to detect the ISO 3166-1 alpha-2 name
and to select the correct conformance test limits (CTL) for a country. If
the country isn't available and it is still programmed in the EEPROM then
it will cause an error and stop the initialization with:

  Invalid EEPROM contents

The current CTL mappings for this country are:

* 2.4GHz: ETSI
* 5GHz: ETSI

Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>