On at least some platforms (observed with a Raspberry Pi), any attempt
to perform USB transfers via EFI_USB_IO_PROTOCOL during EFI shutdown
will lock up the system.  This is quite probably due to the already
documented failure of all EFI timers when ExitBootServices() is
called: see e.g. commit 5cf5ffea2 "[efi] Work around temporal anomaly
encountered during ExitBootServices()".

Work around this problem by refusing to poll endpoints if shutdown is
in progress, and by immediately failing any attempts to enqueue new
transfers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>