Fix compilation warning.

size_t is unsigned; the #defined constant was treated as an integer,
which caused the following warning:

In file included from cores\nRF5\Uart.cpp:21:0:
cores\nRF5\Arduino.h: In instantiation of
```C++
  decltype ( ((b < a) ? b :  a) )
   min(const T&, const L&)
   [with
    T = unsigned int;
    L = int;
    decltype (((b < a) ? b :  a)) = unsigned int
   ]:
```

cores\nRF5\Uart.cpp:228:54: required from here
cores\nRF5\Arduino.h:92:15: warning:
comparison between signed and unsigned integer expressions [-Wsign-compare]
     return (b < a) ? b : a;

cores/nRF5/Uart.cpp

@@ -225,7 +225,7 @@ size_t Uart::write(const uint8_t *buffer, size_t size)
   do
   {
     size_t remaining = size - sent;
-    size_t txSize = min(remaining, SERIAL_BUFFER_SIZE);
+    size_t txSize = min(remaining, (size_t)SERIAL_BUFFER_SIZE);
 
     xSemaphoreTake(_mutex, portMAX_DELAY);