RB3MidiGuitar.cpp: Support 5-Fret play

Rock Band 3 has support for playing the regular guitar mode with the
Pro Guitar. While the real MPA does not support this, the game accepts
it just fine so there's no harm in emulating it.

rpcs3/Emu/Io/RB3MidiGuitar.cpp

@@ -288,6 +288,49 @@ void usb_device_rb3_midi_guitar::write_state(u8* buf)
 	buf[10] = button_state.string_velocities[4];
 	buf[9] = button_state.string_velocities[5];
 
+	// encode frets for playing 5 fret on the pro guitar
+	// this actually isn't done by the real MPA, but Rock Band 3 allows this
+	// so there's no harm in supporting it.
+	for (u8 i : button_state.frets)
+	{
+		switch (i)
+		{
+			case 1:
+			case 6:
+			case 13:
+				buf[9] |= 0b1000'0000;
+				break;
+			case 2:
+			case 7:
+			case 14:
+				buf[10] |= 0b1000'0000;
+				break;
+			case 3:
+			case 8:
+			case 15:
+				buf[11] |= 0b1000'0000;
+				break;
+			case 4:
+			case 9:
+			case 16:
+				buf[12] |= 0b1000'0000;
+				break;
+			case 5:
+			case 10:
+			case 17:
+				buf[13] |= 0b1000'0000;
+				break;
+			default:
+				break;
+		}
+
+		// enable the solo bit for frets >= 13
+		if (i >= 13)
+		{
+				buf[8] |= 0b1000'0000;
+		}
+	}
+
 	// encode tilt sensor/sustain_pedal
 	if (button_state.tilt_sensor || button_state.sustain_pedal)
 	{