Fix a bug in Mellanox plugin RPC caused by secgroup RPC refactoring

SecurityGroupsRpcCallback RPC refactoring patch (commit 2781fce238)
moves get_port_from_device method from plugin RPC callback class to
plugin layer, but Mellanox plugin RPC callback was forgot to update
and as a result RPC calls from agents to Mellanox plugin fails.

Change-Id: I1aa82ac00c16a53a59a93087f0ca4ef281ee3f2b
Related-Bug: #1359416
Merge "Fix a bug in Mellanox plugin RPC caused by secgroup RPC refactoring"