atm: atmtcp: Constify atmtcp_v_dev_ops

The only usage of atmtcp_v_dev_ops is to pass its address to
atm_dev_register() which takes a pointer to const, and comparing its
address to another address, which does not modify it. Make it const to
allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>