ethernet: fix VLAN RelatedInterfaces URL missing parent interface name

The format string was missing the second {} placeholder, causing
Links/RelatedInterfaces to not point to the parent interface.

Tested: POST VLAN, GET vlan interface shows correct parent URI in
Links/RelatedInterfaces.

Change-Id: I2153e5641a9bc337d17b186d3bfbe73ce5cab91b
Signed-off-by: Rashid MP <rashidmp@ami.com>