Fix axis char printing

Marlin/src/core/serial.h

@@ -144,6 +144,7 @@ void SERIAL_ECHO(T x) { SERIAL_IMPL.print(x); }
 typedef struct SerialChar { char c; SerialChar(char n) : c(n) { } } serial_char_t;
 inline void SERIAL_ECHO(serial_char_t x) { SERIAL_IMPL.write(x.c); }
 #define AS_CHAR(C) serial_char_t(C)
+#define AS_DIGIT(C) AS_CHAR('0' + (C))
 
 // SERIAL_ECHO_F prints a floating point value with optional precision
 inline void SERIAL_ECHO_F(EnsureDouble x, int digit=2) { SERIAL_IMPL.print(x, digit); }