[efi] Avoid infinite loops when asked to stop non-existent devices

Calling EDK2's OpenProtocol() with attributes BY_DRIVER|EXCLUSIVE will
call DisconnectController() in a loop to attempt to dislodge any
existing openers with attributes BY_DRIVER.  The loop will continue
indefinitely until either no such openers remain, or until
DisconnectController() returns an error.

If our driver binding protocol's Stop() method is ever called to
disconnect a device that we are not in fact driving, then return
EFI_DEVICE_ERROR rather than EFI_SUCCESS, in order to break this
potentially infinite loop.

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

src/interface/efi/efi_driver.c

 	if ( ! efidev ) {
 		DBGCP ( device, "EFIDRV %s is not started\n",
 			efi_handle_name ( device ) );
-		return 0;
+		return EFI_DEVICE_ERROR;
 	}
 
 	/* Stop this device */