Errors may occur inside efi_smbios_register().

- Return a status code.
- Remove unused variables.
- Use constants where applicable.
Suggested-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>