staging: dwc2: check for null pointer before dereferencing it

We were testing qtd->urb pointer for null after we had already
dereferenced it

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>