[media] smiapp: Don't give the source sub-device a temporary name

The source sub-device's name will be overwritten shortly. Don't give it a
name in the meantime.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>