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

UPSTREAM: 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()

Change-Id: Ie541e0390fd5475852ede4216b2b90d7a87f5430
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>
Signed-off-by: Huang, Tao <huangtao@rock-chips.com>
(cherry picked from commit b0d659022e5c96ee5c4bd62d22d3da2d66de306b)