Ignore clock and active sensing on serial MIDI

Fixes #416

I'm not entirely sure this is the correct way to fix it. But it does
seem to fix the problem I was seeing. I can now completely mash the
keyboard and no notes will get stuck on.

src/serialmididevice.cpp

@@ -109,6 +109,10 @@ void CSerialMIDIDevice::Process (void)
 			}
 			continue;
 		}
+		else if(uchData == 0xF8 || uchData == 0xFE)
+		{
+			continue;
+		}
 		else
 		{
 			switch (m_nSerialState)