Bluetooth: 6lowpan: Make variable header_ops constant

Static variable header_ops, of type header_ops, is used only once, when
it is assigned to field header_ops of a variable having type net_device.
This corresponding field is declared as const in the definition of
net_device. Hence make header_ops constant as well to protect it from
unnecessary modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -583,7 +583,7 @@ static const struct net_device_ops netdev_ops = {
 	.ndo_start_xmit		= bt_xmit,
 };
 
-static struct header_ops header_ops = {
+static const struct header_ops header_ops = {
 	.create	= header_create,
 };