Small fix to handle the fact that ovsdb sometimes returns ofport of -1

It turns out that sometimes ovsdb returns an ofport of -1 to indicate that
there is no associated ofport value.  We should represent that by *not*
having an ofport value

Change-Id: I2a28a34333ff1b92728286672499714079346444
Signed-off-by: Ed Warnicke <eaw@cisco.com>