efi_loader: Fix -Wextra warning for EFI TCG2

Compiling with -Wextra produces a number of (harmless) warnings for the EFI
TCG2 protocol.  Let's fix those in case we ever enable the flag on the EFI
subsystem

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>