usb-ehci: Fix an assert whenever isoc transfers are used

hcd-ehci.c is missing an usb_packet_init() call for the ipacket UsbPacket
it uses for isoc transfers, triggering an assert (taking the entire vm down)
in usb_packet_setup as soon as any isoc transfers are done by a high speed
USB device.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-ehci.c

@@ -2581,6 +2581,7 @@ static int usb_ehci_initfn(PCIDevice *dev)
     s->async_bh = qemu_bh_new(ehci_async_bh, s);
     QTAILQ_INIT(&s->aqueues);
     QTAILQ_INIT(&s->pqueues);
+    usb_packet_init(&s->ipacket);
 
     qemu_register_reset(ehci_reset, s);