char: virtio: constify attribute_group structures.

attribute_group are not supposed to change at runtime. All functions
working with attribute_group provided by <linux/sysfs.h> work with
const attribute_group. So mark the non-const structs as const.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/virtio_console.c

@@ -1308,7 +1308,7 @@ static struct attribute *port_sysfs_entries[] = {
 	NULL
 };
 
-static struct attribute_group port_attribute_group = {
+static const struct attribute_group port_attribute_group = {
 	.name = NULL,		/* put in device directory */
 	.attrs = port_sysfs_entries,
 };