[media] media-device.h: fix compiler warning

Fix these compiler warnings:

media-git/include/media/media-device.h: In function 'media_device_pci_init':
media-git/include/media/media-device.h:610:9: warning: 'return' with a value, in function returning void
  return NULL;
         ^
media-git/include/media/media-device.h: In function '__media_device_usb_init':
media-git/include/media/media-device.h:618:9: warning: 'return' with a value, in function returning void
  return NULL;
         ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>