lora/lorawan: refactor shared functionality for ChannelUS, etc into embedded type

named channel, do the same for RegionSettings, and then change RegionSettings to
just Settings to avoid the redundant naming.

Signed-off-by: deadprogram <ron@hybridgroup.com>

lora/lorawan/region/settings.go

@@ -0,0 +1,25 @@
+package region
+
+type Settings interface {
+	JoinRequestChannel() Channel
+	JoinAcceptChannel() Channel
+	UplinkChannel() Channel
+}
+
+type settings struct {
+	joinRequestChannel Channel
+	joinAcceptChannel  Channel
+	uplinkChannel      Channel
+}
+
+func (r *settings) JoinRequestChannel() Channel {
+	return r.joinRequestChannel
+}
+
+func (r *settings) JoinAcceptChannel() Channel {
+	return r.joinAcceptChannel
+}
+
+func (r *settings) UplinkChannel() Channel {
+	return r.uplinkChannel
+}