Eliminate the "q_ptr->" call by using Q_Q(QSerialPort)

Unfortunately, the q_ptr data member cannot yet be deleted from the internal
data class because QIODevicePrivate would need to be inherited then. That is not
a simple change considering the Qt4 support. Hence, that part of the logic is
put on hold, but can be expected in an upcoming change at some point, soon.

Change-Id: Ic8ba9621dd647f4afa1b91d01c858c836d1cc0c8
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>