Optimize access to the write buffer

Earlier for writing in writeBuffer was used the
QSerialPortPrivate::writeToBuffer() method which contained
a similar code for each platform.

Therefore it is reasonable to delete this method, and to move
its common functionality to the QSerialPort::writeData(). Also
all platform-dependent code of start of data transfer move to
the QSerialPortPrivate::startWriting() method.

Change-Id: I1423723fd69c05df974f8ba597e3dd71d5a797dd
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>