cros_ec: fix nul-termination for firmware build info

As gcc-8 reports, we zero out the wrong byte:

drivers/platform/chrome/cros_ec_sysfs.c: In function 'show_ec_version':
drivers/platform/chrome/cros_ec_sysfs.c:190:12: error: array subscript 4294967295 is above array bounds of 'uint8_t[]' [-Werror=array-bounds]

This changes the code back to what it did before changing to a
zero-length array structure.

Fixes: a841178445 ("mfd: cros_ec: Use a zero-length array for command data")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benson Leung <bleung@chromium.org>

drivers/platform/chrome/cros_ec_sysfs.c

@@ -185,7 +185,7 @@ static ssize_t show_ec_version(struct device *dev,
 		count += scnprintf(buf + count, PAGE_SIZE - count,
 				   "Build info:    EC error %d\n", msg->result);
 	else {
-		msg->data[sizeof(msg->data) - 1] = '\0';
+		msg->data[EC_HOST_PARAM_SIZE - 1] = '\0';
 		count += scnprintf(buf + count, PAGE_SIZE - count,
 				   "Build info:    %s\n", msg->data);
 	}