Fix define ON OFF conflict with other libraries (arduino-libraries#77)

* Remove conflicting #define

These defines conflict the use of ON with other arduino libraries. It is only used in one place in modbus.c

Author: jahartley

src/libmodbus/modbus.c

@@ -864,7 +864,7 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
 #else
             if (data == 0xFF00 || data == 0x0) {
 #endif
-                mb_mapping->tab_bits[mapping_address] = data ? ON : OFF;
+                mb_mapping->tab_bits[mapping_address] = data ? 1 : 0;
                 memcpy(rsp, req, req_length);
                 rsp_length = req_length;
             } else {

src/libmodbus/modbus.h

@@ -50,13 +50,6 @@ MODBUS_BEGIN_DECLS
 #define TRUE 1
 #endif
 
-#ifndef OFF
-#define OFF 0
-#endif
-
-#ifndef ON
-#define ON 1
-#endif
 
 /* Modbus function codes */
 #define MODBUS_FC_READ_COILS                0x01