Fix G27 button output when using DF shifter

This lets you read the Driving Force shifter with the G27 object. As the detection pins are the same, the two shifters cannot be simply distinguished by the microcontroller.

Author: dmadison

src/SimRacing.cpp

@@ -1247,6 +1247,21 @@ uint16_t LogitechShifterG27::readShiftRegisters() {
 	}
 	digitalWrite(this->pinClock, LOW);
 
+	// edge case: two of the bits (0x8000 and 0x2000) are connected only to
+	// pull-down resistors, and should theoretically never be high. If they,
+	// and all other bits, *are* high, then we are not reading from a shifter
+	// that has shift registers. The "Driving Force" (G29/G920/G923) shifter
+	// has its data output connected to the 'reverse' button through a buffer,
+	// and will report 'high' if the reverse button is pressed no matter how
+	// many times the clock is pulsed.
+	//
+	// QED: we are connected to a "Driving Force" shifter, and not a G27.
+	// That's okay! If we set the state of the 'reverse' button and clear
+	// all others, we can still behave like a G27.
+	if (data == 0xFFFF) {
+		data = (1 << (uint8_t) Button::BUTTON_REVERSE);
+	}
+
 	return data;
 }