Use pgm_read_ptr for tables of pointers

Marlin/src/feature/tmc_util.cpp

                     ext_E2[] PROGMEM = "E2", ext_E3[] PROGMEM = "E3",
                     ext_E4[] PROGMEM = "E4";
   const static char* const tmc_axes[] PROGMEM = { ext_X, ext_Y, ext_Z, ext_X2, ext_Y2, ext_Z2, ext_E0, ext_E1, ext_E2, ext_E3, ext_E4 };
-  serialprintPGM((char*)pgm_read_word(&tmc_axes[axis]));
+  serialprintPGM((char*)pgm_read_ptr(&tmc_axes[axis]));
 }
 
 void _tmc_say_current(const TMC_AxisEnum axis, const uint16_t curr) {