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

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: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 7341ea075c09258b98a1d0efc60efd402cbfc9b4)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>