replace UINT8_MAX by UNUSED_PIN

src/MFRC522.h

@@ -142,6 +142,8 @@ class MFRC522 {
 public:
 	// Size of the MFRC522 FIFO
 	static const byte FIFO_SIZE = 64;		// The FIFO is 64 bytes.
+	// Default value for unused pin
+	static constexpr uint8_t UNUSED_PIN = UINT8_MAX;
 
 	// MFRC522 registers. Described in chapter 9 of the datasheet.
 	// When using SPI all addresses are shifted one bit left in the "SPI address byte" (section 8.1.2.3)