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>