MidiPort: subscribe writable rather than readable port when settings

When loading MidiPort settings, erroneously readable port was subscribed rather
than writable port. This commit fixes the issue.
(cherry picked from commit d981b32480)

src/core/midi/midi_port.cpp

@@ -266,7 +266,7 @@ void midiPort::loadSettings( const QDomElement & _this )
 		{
 			if( it.value() != ( wp.indexOf( it.key() ) != -1 ) )
 			{
-				subscribeReadablePort( it.key() );
+				subscribeWritablePort( it.key() );
 			}
 		}
 		emit writablePortsChanged();