udev-seat: Fail input setup only if no devices are found

Rather than failing if we cannot open any single device fail the input
setup if there are no input devices added.

https://bugs.freedesktop.org/show_bug.cgi?id=64506

src/udev-seat.c

@@ -58,7 +58,7 @@ device_added(struct udev_device *udev_device, struct udev_seat *master)
 	fd = weston_launcher_open(c, devnode, O_RDWR | O_NONBLOCK);
 	if (fd < 0) {
 		weston_log("opening input device '%s' failed.\n", devnode);
-		return -1;
+		return 0;
 	}
 
 	device = evdev_device_create(&master->base, devnode, fd);
@@ -69,7 +69,7 @@ device_added(struct udev_device *udev_device, struct udev_seat *master)
 	} else if (device == NULL) {
 		close(fd);
 		weston_log("failed to create input device '%s'.\n", devnode);
-		return -1;
+		return 0;
 	}
 
 	calibration_values =
@@ -140,6 +140,7 @@ udev_seat_add_devices(struct udev_seat *seat, struct udev *udev)
 			"\t- seats misconfigured "
 			"(Weston backend option 'seat', "
 			"udev device property ID_SEAT)\n");
+		return -1;
 	}
 
 	return 0;