SerialBase.h|cpp [-Wreorder] compiler warning fix

The initialization order was not quite right, so aligning the .cpp
to match order .h.  This hixes the following compiler warning:

```
[Warning] SerialBase.h@351,22: 'mbed::SerialBase::_flow2' will be initialized after [-Wreorder]
[Warning] SerialBase.h@343,22:   'bool mbed::SerialBase::_rx_enabled' [-Wreorder]
[Warning] SerialBase.cpp@26,1:   when initialized here [-Wreorder]

```

Author: Janne Kiiskila

drivers/source/SerialBase.cpp

@@ -32,15 +32,15 @@ SerialBase::SerialBase(PinName tx, PinName rx, int baud) :
 #endif
     _serial(),
     _baud(baud),
+    _rx_enabled(true),
+    _tx_enabled(true),
+    _tx_pin(tx),
+    _rx_pin(rx),
 #if DEVICE_SERIAL_FC
     _flow_type(Disabled),
     _flow1(NC),
-    _flow2(NC),
+    _flow2(NC)
 #endif
-    _rx_enabled(true),
-    _tx_enabled(true),
-    _tx_pin(tx),
-    _rx_pin(rx)
 {
     // No lock needed in the constructor