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/wifi/generic/smbios.c

@@ -28,7 +28,7 @@ static int smbios_write_intel_wifi(struct device *dev, int *handle, unsigned lon
 	/* Intel wifi driver expects this string to be in the table 0x85. */
 	t->str = smbios_add_string(t->eos, "KHOIHGIUCCHHII");
 
-	const int len = t->length + smbios_string_table_len(t->eos);
+	const int len = smbios_full_table_len(&t->header, t->eos);
 	*current += len;
 	*handle += 1;
 	return len;