ieee802154: 6lowpan: make header_ops const

Make this const as it is only stored as a reference in a const field of
a net_device structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan/core.c

@@ -54,7 +54,7 @@
 
 static int open_count;
 
-static struct header_ops lowpan_header_ops = {
+static const struct header_ops lowpan_header_ops = {
 	.create	= lowpan_header_create,
 };