[efi] Match EDK2 numbering for USB ports

The various USB specifications all use one-based numbering for ports.
This scheme is applied consistently across the various relevant
specifications, covering both port numbers that appear on the wire
(i.e. downstream hub port numbers) and port numbers that exist only
logically (i.e. root hub port numbers).

The UEFI specification is ambiguous about the port numbers as used for
the ParentPortNumber field within a USB_DEVICE_PATH structure.  As of
UEFI specification version 2.8 errata B:

- section 10.3.4.5 just states "USB Parent Port Number" with no
  indication of being zero-based or one-based

- section 17.1.1 notes that for the EFI_USB2_HC_PROTOCOL, references
  to PortNumber parameters are zero-based for root hub ports

- section 17.1.1 also mentions a TranslatorPortNumber used by
  EFI_USB2_HC_PROTOCOL, with no indication of being zero-based or
  one-based

- there are no other mentions of USB port numbering schemes.

Experimentation and inspection of the EDK2 codebase reveals that at
least the EDK2 reference implementation will use zero-based numbering
for both root and non-root hub ports when populating a USB_DEVICE_PATH
structure (though will inconsistently use one-based numbering for the
TranslatorPortNumber parameter).

Use zero-based numbering for both root and non-root hub ports when
constructing a USB_DEVICE_PATH in order to match the behaviour of the
EDK2 implementation.

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

src/interface/efi/efi_usb.c

@@ -1100,7 +1100,7 @@ static int efi_usb_install ( struct efi_usb_device *usbdev,
 		usbpath->Header.Type = MESSAGING_DEVICE_PATH;
 		usbpath->Header.SubType = MSG_USB_DP;
 		usbpath->Header.Length[0] = sizeof ( *usbpath );
-		usbpath->ParentPortNumber = usb->port->address;
+		usbpath->ParentPortNumber = ( usb->port->address - 1 );
 	}
 
 	/* Add to list of interfaces */