dumpPortsPrefix is shown multiple times on ovs with many ports

[mstinsky]

Greetings,

while using this library we found that the parsing of the ovs dumpPorts breaks when ovs has a lot of ports because the dumpPortsPrefix "OFPST_PORT reply" can be shown multiple times.

This patch adds a condition to check if we see the dumpPortsPrefix again to continue the loop without parsing it.