Fix cable validator to allow editing of existing cable

The validator as currently implemented, when modifying an existing
cable, ends up getting itself back as a result of the query and
thinking it's a duplicate.

I'm sure more elegant ways to do this but this will prevent it
from detecting itself as a duplicate when modifying.

Bug: T310590
Change-Id: I81703d55a88a9ca6685d9723d6020bcadda6125f