ofagent: Fix an argument mismatch bug in commit 9d13ea88

recently merged commit 9d13ea884bff749b3975acb5eb5630e5aca4a665
had an argument mismatch bug which causes the following crash.

    TypeError: _get_ports() takes exactly 2 arguments (1 given)

this commit fixes it.

Closes-Bug: #1336175
Change-Id: I46bb1fc1aebf06274bb226014e4a9bc5a83f98ac
Merge "ofagent: Fix an argument mismatch bug in commit 9d13ea88"