Fix return from void function

While gcc on Linux (Travis CI) throws a warning, but otherwise ignores
this return value from void function, clang on OSX throws a fit.

cli/x52_cli.c

@@ -333,8 +333,6 @@ static void do_help(const struct command_handler *cmd)
 
         printf("\nWARNING: raw command may damage your device\n\n");
     }
-
-    return 0;
 }
 
 int main(int argc, char **argv)