[sam] Fix compiler warnings when using GpioUnused in SoftwareGpioPort

Author: chris-durand

src/modm/platform/gpio/sam/unused.hpp

@@ -31,7 +31,7 @@ class GpioUnused : public ::modm::GpioIO
 	using Type = GpioUnused;
 	static constexpr bool isInverted = false;
 	static constexpr PortName port = PortName(-1);
-	static constexpr uint8_t pin = uint8_t(-1);
+	static constexpr uint8_t pin = 0;
 	static constexpr uint32_t mask = 0;
 
 	// GpioOutput