QSerialPort::bindableStopBits has a wrong return value

It was overlooked when initially introducing the bindable property support.
The property has type StopBits, so that should also be the return type of the bindableStopBits method.
Currently bool is returned, which compiles and somehow works due to implicit conversions.

The method is a part of the public API, so we can only get rid of it using QT_REMOVED_SINCE.

The idea is that we should introduce a new method bindableStopBitsFixed, and use it in the Q_PROPERTY() macro. Not sure if we can pick such change to older branches, though...