Fix 60 byte DHCP leak per ::begin

LWIP DHCP needs a separate _cleanup call to free the preallocated DHCP
packet PBUF or it will leak 60 bytes every <Ethernet>::end.  Add the call
on LwipIntfDev::end

Author: earlephilhower

libraries/lwIP_Ethernet/src/LwipIntfDev.h

@@ -463,6 +463,7 @@ void LwipIntfDev<RawDev>::end() {
     if (_started) {
         if (_isDHCP) {
             dhcp_stop(&_netif);
+            dhcp_cleanup(&_netif);
         }
         if (_intrPin < 0) {
             __removeEthernetPacketHandler(_phID);