efi_loader: fix boot_service_capability_min calculation

TCG EFI Protocol Specification requires to the input
ProtocolCapability.Size < size of the EFI_TCG2_BOOT_SERVICE_CAPABILITY
up to and including the vendor ID field.
Current implementation does different calculation, let's fix it.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>