Fix handling of writing at using of waitForReadyRead()

The method waitForReadyRead() shall call of the write sequence since the
signals from the startAsyncWriteTimer will not be handled inside of
waitForReadyRead() without event-loop.

Tested on Windows 8 with the on-board and USB serial ports, using Qt5.

Task-number: QTBUG-40793
Change-Id: I7806f16a4df30c4ec1643d8f696ad5761decd30a
Reviewed-by: Patrick Noffke <patrick.noffke@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>