usb: dwc2: fix dereference before NULL check

In a couple of places, we were checking qtd->urb for NULL after
we had already dereferenced it. Fix this by moving the check to
before the dereference.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>