[media] siano: avoid a linkedit error if !MC

If the media controller (MC) is not enabled, it will compile
fine, but will fail at the linkedition:

ERROR: "media_device_unregister" [drivers/media/usb/siano/smsusb.ko] undefined!

Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/usb/siano/smsusb.c

···
 	if (rc < 0) {
 		pr_err("smscore_register_device(...) failed, rc %d\n", rc);
 		smsusb_term_device(intf);
+#ifdef CONFIG_MEDIA_CONTROLLER_DVB
 		media_device_unregister(mdev);
+#endif
 		kfree(mdev);
 		return rc;
 	}