greybus: quick fix for sysfs serial number

Let the serial number attribute have its own is_visible function.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/sysfs.c

@@ -156,9 +156,15 @@ static struct attribute *serial_number_attrs[] = {
 	NULL,
 };
 
+static umode_t serial_number_is_visible(struct kobject *kobj,
+					struct attribute *a, int n)
+{
+	return a->mode;
+}
+
 static struct attribute_group serial_number_attr_grp = {
 	.attrs =	serial_number_attrs,
-	.is_visible =	function_attrs_are_visible,
+	.is_visible =	serial_number_is_visible,
 };