mac80211-hwsim: Add HWSIM_CMD_GET_RADIO command

HWSIM_CMD_GET_RADIO returns information about a specific radio id or
all of them in response to a dump. Create the netlink skb or use the
one provided by the dump functionality. Use the existing attribute
appending function to fill in the same attributes when creating a
new hwsim radio.

Save alpha2 and struct ieee80211_regdomain in the hwsim data or else
they will be lost in the depths of regulatory infrastructure.

Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>