xgbe: constify get_netdev_ops and get_ethtool_ops

Casting away const is bad practice. Since this is ARM specific driver
don't have hardware actually test this.

Having getter functions for ops is really unnecessary code bloat, but
not going to touch that.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>