dwc_otg: fix bug with port_addr assignment for single-TT hubs

See https://github.com/raspberrypi/linux/issues/2734

The "Hub Port" field in the split transaction packet was always set
to 1 for single-TT hubs. The majority of single-TT hub products
apparently ignore this field and broadcast to all downstream enabled
ports, which masked the issue. A subset of hub devices apparently
need the port number to be exact or split transactions will fail.