Avoid an iteration in flush function

The loop test ought to iterate again to end the loop but this is
only required if the socket contained more data than read.

src/modbus-tcp.c

@@ -354,7 +354,7 @@ int _modbus_tcp_flush(modbus_t *ctx)
         if (ctx->debug && rc != -1) {
             printf("\n%d bytes flushed\n", rc);
         }
-    } while (rc > 0);
+    } while (rc == MODBUS_TCP_MAX_ADU_LENGTH);
 
     return rc;
 }