Don't use IPAddress::_address from EthernetClass

EthernetClass is a friend class of IPAddress, so it is allowed to use
its _address attribute directly. However, it should be using
IPAddress::raw_address() instead, like all the other friend classes do.

This changes allows changing the _address attribute to fix some warnings
next.

Author: matthijskooijman

Diff for: libraries/Ethernet/src/Ethernet.cpp

@@ -62,9 +62,9 @@ void EthernetClass::begin(uint8_t *mac, IPAddress local_ip, IPAddress dns_server
 {
   W5100.init();
   W5100.setMACAddress(mac);
-  W5100.setIPAddress(local_ip._address);
-  W5100.setGatewayIp(gateway._address);
-  W5100.setSubnetMask(subnet._address);
+  W5100.setIPAddress(local_ip.raw_address());
+  W5100.setGatewayIp(gateway.raw_address());
+  W5100.setSubnetMask(subnet.raw_address());
   _dnsServerAddress = dns_server;
 }