efi_loader: efi_smbios_register should have a return value

Errors may occur inside efi_smbios_register().

- Return a status code.
- Remove unused variables.
- Use constants where applicable.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>