net: atm: constify in_cache_ops and eg_cache_ops structures

The in_cache_ops and eg_cache_ops structures are never modified, so declare
them as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>