util/nvmutil: use strlen instead of strnlen

strnlen isn't available on some older unices.

we already know the string will be null-terminated,
because it comes from argv, so runaway reads are
extremely unlikely (read: impossible).

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -359,7 +359,7 @@ parse_mac_string(void)
 {
 	int mac_pos;
 
-	if (strnlen(mac, 20) != 17)
+	if (strlen(mac) != 17)
 		err(EINVAL, "MAC address is the wrong length");
 
 	for (mac_pos = 0; mac_pos < 16; mac_pos += 3)