Make the QSerialPort::flush() non-blocking

According to documentation, the flush() method shall be non-blocking.

Tested on Linux 64-bit with auto-tests with the on-board and the USB
serial ports for Qt4 and then Qt5.

Note: On Windows the flush() method still does not work.

Change-Id: Iaee80361e59e0c281206ca24c817a446cdbf6ed1
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>