Fix signed/unsigned comparison warning in SerialBase.cpp

Silence the following compiler warning:

Compile: SerialBase.cpp
[Warning] SerialBase.cpp@34,23: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tony Wu <[email protected]>

Author: tung7970

hal/common/SerialBase.cpp

@@ -31,7 +31,7 @@ SerialBase::SerialBase(PinName tx, PinName rx) :
                                                 _serial(), _baud(9600) {
     // No lock needed in the constructor
 
-    for (int i = 0; i < sizeof _irq / sizeof _irq[0]; i++) {
+    for (size_t i = 0; i < sizeof _irq / sizeof _irq[0]; i++) {
         _irq[i].attach(donothing);
     }