libsa: smbios_probe(): Strictly obey specified entry point address

When such an address is known, do not search elsewhere, contrary to what
we are doing on non-EFI boot (see SMBIOS_START and SMBIOS_LENGTH).

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49291