[media] smiapp: Read link-frequencies property from the endpoint node

The documentation stated that the link-frequencies property belongs to the
endpoint node, not to the device's of_node. Fix this.

There are no DT board descriptions using the driver yet, so a fix in the
driver is sufficient.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>