Merge pull request #6111 from benlye/benlye-patch-echo_command

Fix type conversion issue with echo_command

Marlin/Marlin_main.cpp

   extern void digipot_i2c_init();
 #endif
 
-inline void echo_command(char * const cmd) {
+inline void echo_command(const char* cmd) {
   SERIAL_ECHO_START;
   SERIAL_ECHOPAIR(MSG_ENQUEUEING, cmd);
   SERIAL_CHAR('"');