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>
diff --git a/drivers/usb/dwc2/hcd.h b/drivers/usb/dwc2/hcd.h
index 89fa26c..7758bfb 100644
--- a/drivers/usb/dwc2/hcd.h
+++ b/drivers/usb/dwc2/hcd.h
@@ -552,6 +552,7 @@
 {
 	list_del(&qtd->qtd_list_entry);
 	kfree(qtd);
+	qtd = NULL;
 }
 
 /* Descriptor DMA support functions */