usb: dwc2: Fix issues in dwc2_complete_non_isoc_xfer_ddma()

Fixes a static analysis issue in dwc2_complete_non_isoc_xfer_ddma(). The
qtd was being passed to a function after being freed. It was not being
used in the function so this doesn't fix any bugs. But it fixes up the
warning and makes the code safer by setting qtd to NULL and not using it
at all.

Reported-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>