efi-stub: Fix get_efi_config_table on mixed-mode setups

Fix get_efi_config_table using the wrong structs when booting a
64 bit kernel on 32 bit firmware.

Fixes: 82d736ac

 ("Abstract out support for locating an EFI config table")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-By: Matthew Garrett <mjg59@google.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>