cfg80211: add helper reg_get_regdomain() function

Add helper function that will return regdomain.
Follow the driver's regulatory domain, if present,
unless a country IE has been processed or a user
wants to help compliance further.

Signed-off-by: Janusz Dziedzic <janusz.dziedzi@tieto.com>
[remove useless reg variable]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>