drivers: net: xgene: constify xgene_mac_ops and xgene_port_ops structures

The xgene_mac_ops and xgene_port_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>