moves static declaration out of method

because of a known bug, the declaration of DhcpClass has to be outside
the EthernetClass::begin(*mac) Method. Only if it is used with DHCP this
 error occurs.

More details: esp8266/Arduino#500

Author: ofesseler

lib/Ethernet/src/Ethernet.cpp

@@ -8,9 +8,12 @@ uint8_t EthernetClass::_state[MAX_SOCK_NUM] = {
 uint16_t EthernetClass::_server_port[MAX_SOCK_NUM] = { 
   0, 0, 0, 0 };
 
+// moved out of method scope, because of this issue:
+// https://github.com/esp8266/Arduino/issues/500
+static DhcpClass s_dhcp;
+
 int EthernetClass::begin(uint8_t *mac_address, unsigned long timeout, unsigned long responseTimeout)
 {
-  static DhcpClass s_dhcp;
   _dhcp = &s_dhcp;