firmware: dmi: Fortify entry point length checks

Ensure that the SMBIOS entry point is long enough to include all the
fields we need. Otherwise it is pointless to even attempt to verify
its checksum.

Also fix the maximum length check, which is technically 32, not 31.
It does not matter in practice as the only valid values are 31 (for
SMBIOS 2.x) and 24 (for SMBIOS 3.x), but let's still have the check
right in case new fields are added to either structure in the future.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/lkml/20220823094857.27f3d924@endymion.delvare/T/