ehci: Fix NULL ptr deref when unplugging an USB dev with an iso stream active

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
(cherry picked from commit 7ce86aa1)

Conflicts:

	hw/usb/hcd-ehci.c

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