cfg80211: don't require RTNL held for regdomain reads

The whole code is set up to allow RCU reads of this data, but
then uses rtnl_dereference() which requires the RTNL. Convert
it to rcu_dereference_rtnl() which makes it require only RCU
or the RTNL, to allow RCU-protected reading of the data.

Reviewed-by: Coelho, Luciano <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>