ekf2: multi-ekf force initial selection (and sensor_selection publication)

- depending on the particular sensor configuration and startup timing we were getting an initial sensor_selection publication without valid accel & gyro device ids (only applies to Multi-EKF)
 - fix faulty accel and gyro device ids
 - initialize unused test ratio fields to NAN for logging clarity

src/modules/ekf2/EKF2.cpp

@@ -1557,10 +1557,10 @@ int EKF2::task_spawn(int argc, char *argv[])
 
 			if (inst) {
 				_ekf2_selector.store(inst);
-				inst->Start();
 
 			} else {
-				PX4_ERR("Failed to start EKF2 selector");
+				PX4_ERR("Failed to create EKF2 selector");
+				return PX4_ERROR;
 			}
 		}
 
@@ -1604,6 +1604,7 @@ int EKF2::task_spawn(int argc, char *argv[])
 								ekf2_inst->ScheduleNow();
 								success = true;
 								multi_instances_allocated++;
+								_ekf2_selector.load()->ScheduleNow();
 
 							} else {
 								PX4_ERR("instance %d alloc failed", instance);