dwc_otg: fix split transaction data toggle handling around dequeues

See https://github.com/raspberrypi/linux/issues/1709

Fix several issues regarding endpoint state when URBs are dequeued
- If the HCD is disconnected, flush FIQ-enabled channels properly
- Save the data toggle state for bulk endpoints if the last transfer
  from an endpoint where URBs were dequeued returned a data packet
- Reset hc->start_pkt_count properly in assign_and_init_hc()