ath6kl: add support for changing contry code

To make it possible to change the country code from user space via nl80211
add handler for reg_notifier. The feature is only enabled when built
time option CONFIG_ATH6KL_REGDOMAIN is enabled, which again depends on
CFG80211_CERTIFICATION_ONUS for certication purposes.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>