l3_db: not use L2 plugin _get_port unnecessarily

l3_db: not use L2 plugin _get_port unnecessarily

This patch is clean up to prevent future breakage by eliminating
potentially dangerous code.

l3_db uses L2 plugin _get_port method unnecessarily instead of get_port.
It's dangerous because _get_port returns ORM db object which allows
the caller to update db rows directly. So the caller of _get_port may
update port db without notifying L2 plugin unintentionally.
In that case, L2 plugin or ML2 mechanism driver will be confused.
This patch replace _get_port with get_port method where possible.

Change-Id: I5a23f6cac5ea359645e6947fd69978f060c4ba97
Related-Bug: #1475093