[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 <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>