usb-ehci: fix ehci_child_detach

Looks like a cut+paste bug from ehci_detach.  When the device itself is
detached from a ehci port (ehci_detach op) we have to clear the
device pointer for the companion port too.  When a device gets removed
from a downstream port of a usb hub (ehci_child_detach op) the ehci port
where the usb hub is plugged in is not affected.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-ehci.c

@@ -797,7 +797,6 @@ static void ehci_child_detach(USBPort *port, USBDevice *child)
     if (portsc & PORTSC_POWNER) {
         USBPort *companion = s->companion_ports[port->index];
         companion->ops->child_detach(companion, child);
-        companion->dev = NULL;
         return;
     }