SerialLink: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

src/comm/SerialLink.h

@@ -52,12 +52,12 @@ public:
     Q_PROPERTY(QString  portDisplayName READ portDisplayName                                NOTIFY portDisplayNameChanged)
     Q_PROPERTY(bool     usbDirect       READ usbDirect          WRITE setUsbDirect          NOTIFY usbDirectChanged)        ///< true: direct usb connection to board
 
-    int  baud()         { return _baud; }
-    int  dataBits()     { return _dataBits; }
-    int  flowControl()  { return _flowControl; }    ///< QSerialPort Enums
-    int  stopBits()     { return _stopBits; }
-    int  parity()       { return _parity; }         ///< QSerialPort Enums
-    bool usbDirect()    { return _usbDirect; }
+    int  baud() const        { return _baud; }
+    int  dataBits() const    { return _dataBits; }
+    int  flowControl() const { return _flowControl; }    ///< QSerialPort Enums
+    int  stopBits() const    { return _stopBits; }
+    int  parity() const      { return _parity; }         ///< QSerialPort Enums
+    bool usbDirect() const   { return _usbDirect; }
 
     const QString portName          () { return _portName; }
     const QString portDisplayName   () { return _portDisplayName; }