SMBIOS: Introduce smbios_full_table_len function

Introduce the `smbios_full_table_len` function to consolidate table
length calculation. The case where the length of a table equals the
length of the structure happens when a table has no strings.

Change-Id: Ibc60075e82eb66b5d0b7132b16da000b153413f9
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55909
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>

src/drivers/elog/elog.c

@@ -626,7 +626,7 @@ int elog_smbios_write_type15(unsigned long *current, int handle)
 	t->log_type_descriptors = 0;
 	t->log_type_descriptor_length = 2;
 
-	const int len = sizeof(*t);
+	const int len = smbios_full_table_len(&t->header, t->eos);
 	*current += len;
 	return len;
 }