usb: assert on calling usb_attach(port, NULL) on a port without a dev

with the "usb-ehci: cleanup port reset handling" patch in place no callers
are calling usb_attach(port, NULL) for a port where port->dev is NULL.

Doing that makes no sense as that causes the port detach op to get called
for a port with nothing attached. Add an assert that port->dev != NULL when
dev == NULL, and remove the check for not having a port->dev in the dev == NULL
case.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb.c

@@ -40,12 +40,11 @@ void usb_attach(USBPort *port, USBDevice *dev)
     } else {
         /* detach */
         dev = port->dev;
+        assert(dev);
         port->ops->detach(port);
-        if (dev) {
-            usb_send_msg(dev, USB_MSG_DETACH);
-            dev->port = NULL;
-            port->dev = NULL;
-        }
+        usb_send_msg(dev, USB_MSG_DETACH);
+        dev->port = NULL;
+        port->dev = NULL;
     }
 }