usb: xhci: Free leaked command in case xhci_queue_stop_endpoint fails

commit e3020f38 ("xhci-hub: Handle error condition with
xhci_stop_device") added error checking to xhci_stop_device()
if xhci_queue_stop_endpoint() returns an error. However, the
command structure allocated was not getting freed in that case.
Fix the leak.

Change-Id: Ibc28e32c9e2d402db0c47bfe759dd442d3ab184f
Signed-off-by: Jack Pham <jackp@codeaurora.org>