Refactor W5500 driver, add arduino w5500 example

mongoose.h

@@ -1423,7 +1423,7 @@ void mg_rpc_list(struct mg_rpc_req *r);
 
 
 struct mip_driver {
-  void (*init)(uint8_t *mac, void *data);           // Initialise driver
+  bool (*init)(uint8_t *mac, void *data);           // Initialise driver
   size_t (*tx)(const void *, size_t, void *data);   // Transmit frame
   size_t (*rx)(void *buf, size_t len, void *data);  // Receive frame (polling)
   bool (*up)(void *data);                           // Up/down status
@@ -1440,13 +1440,14 @@ void mip_init(struct mg_mgr *, struct mip_cfg *, struct mip_driver *, void *);
 
 extern struct mip_driver mip_driver_stm32;
 extern struct mip_driver mip_driver_enc28j60;
+extern struct mip_driver mip_driver_w5500;
 
 // Drivers that require SPI, can use this SPI abstraction
 struct mip_spi {
   void *spi;                        // Opaque SPI bus descriptor
-  uint8_t (*txn)(void *, uint8_t);  // SPI transaction: write 1 byte, read reply
   void (*begin)(void *);            // SPI begin: slave select low
   void (*end)(void *);              // SPI end: slave select high
+  uint8_t (*txn)(void *, uint8_t);  // SPI transaction: write 1 byte, read reply
 };
 
 #ifdef __cplusplus