usb: dwc2: host: Setting qtd to NULL after freeing it

This is safety change added while doing slub debugging.

Affected functions:
dwc2_hcd_qtd_unlink_and_free()
_dwc2_hcd_urb_enqueue()

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>