atm: make atmdev_ops const

Make these structures const as they are either passed to the function
atm_dev_register having the corresponding argument as const or stored in
the ops field of a atm_dev structure, which is also const.
Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>