smbios: Apply the v2.1's length fixup only on a 32-bit entry point

Only allow the length tolerance (0x1e instead of 0x1f) for a 32-bit
entry point, as there was no 64-bit entry point in the erroneous SMBIOS
v2.1 standard and assigning the length with 0x1f does not make sense in
this case.

While here, fix accessing the major/minor versions via 'eps' even in the
64-bit entry point case (not causing any practical problem thus far as
the entry point length is greater than any SMBIOS revisions in
existence, so the comparison guarding the fixup would not pass).

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation