make get_key() result const char *, so that it is not modified

server/drivers/CFontz633.c

@@ -439,7 +439,7 @@ CFontz633_flush (Driver *drvthis)
 /*
  * Return one char from the KeyRing
  */
-MODULE_EXPORT char *
+MODULE_EXPORT const char *
 CFontz633_get_key (Driver *drvthis)
 {
 	// PrivateData *p = drvthis->private_data;