Allow MAX_SOCK_NUM to be overriden by previous define

Author: Flole998

src/Ethernet.h

@@ -33,11 +33,13 @@
 // up to 4 sockets.  W5200 & W5500 can have up to 8 sockets.  Several bytes
 // of RAM are used for each socket.  Reducing the maximum can save RAM, but
 // you are limited to fewer simultaneous connections.
+#ifndef MAX_SOCK_NUM
 #if defined(RAMEND) && defined(RAMSTART) && ((RAMEND - RAMSTART) <= 2048)
 #define MAX_SOCK_NUM 4
 #else
 #define MAX_SOCK_NUM 8
 #endif
+#endif
 
 // By default, each socket uses 2K buffers inside the WIZnet chip.  If
 // MAX_SOCK_NUM is set to fewer than the chip's maximum, uncommenting