tty: serial: constify sunhv_ops structs

Constifies sunhv_ops structures in tty's serial
driver since they are not modified after their
initialization.

Detected and found using Coccinelle.

Suggested-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>