OF-Config: return 'other', not 'invalid', unknown port rates

This closes FlowForwarding/LINC-Switch#227.
Handle empty feature list in linc_ofconfig:rate/1

This closes FlowForwarding/LINC-Switch#226.
Merge pull request #229 from legoscia/of-config-port-rate

OF-Config: return 'other', not 'invalid', unknown port rates