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]

```

Additional fix to cover also the DEVICE_SERIAL_FC case.

Author: Janne Kiiskila

drivers/SerialBase.h

@@ -336,21 +336,18 @@ class SerialBase : private NonCopyable<SerialBase> {
     bool _tx_asynch_set;
     bool _rx_asynch_set;
 #endif
-
+#if DEVICE_SERIAL_FC
+    Flow             _flow_type;
+    PinName          _flow1;
+    PinName          _flow2;
+#endif
     serial_t         _serial;
     Callback<void()> _irq[IrqCnt];
     int              _baud;
     bool             _rx_enabled;
     bool             _tx_enabled;
     const PinName    _tx_pin;
     const PinName    _rx_pin;
-
-#if DEVICE_SERIAL_FC
-    Flow             _flow_type;
-    PinName          _flow1;
-    PinName          _flow2;
-#endif
-
 #endif
 };
 

drivers/source/SerialBase.cpp

@@ -30,13 +30,13 @@ SerialBase::SerialBase(PinName tx, PinName rx, int baud) :
     _rx_callback(NULL), _tx_asynch_set(false),
     _rx_asynch_set(false),
 #endif
-    _serial(),
-    _baud(baud),
 #if DEVICE_SERIAL_FC
     _flow_type(Disabled),
     _flow1(NC),
     _flow2(NC),
 #endif
+    _serial(),
+    _baud(baud),
     _rx_enabled(true),
     _tx_enabled(true),
     _tx_pin(tx),