WinPE has been observed to call PXENV_UNDI_SHUTDOWN but not
PXENV_STOP_UNDI.  This means that Hermon hardware is left partially
active (firmware running and one event queue mapped) when WinPE starts
up, which can cause a Blue Screen of Death.

Fix by ensuring that the hardware is left quiescent (with the firmware
stopped) when no interfaces are open.

Reported-by: Itay Gazit <itayg@mellanox.co.il>
Signed-off-by: Michael Brown <mcb30@ipxe.org>