comedi_test: fix test_cmd_write_fast_1chan() for big buffer

If the comedi data buffer is larger than the total size of the data
being written to the buffer in first test_cmd_write_fast_1chan(), the
first loop iterates endlessly writing 0 bytes to the buffer.  Break out
of the first loop once the required amount of data has been written.

testing/cmd_1.c

@@ -211,8 +211,10 @@ int test_cmd_write_fast_1chan(void)
 		if(ret<wc){
 			go = 0;
 		}
-
 		total += ret;
+		if(total >= num_bytes){
+			go = 0;
+		}
 		if(verbose)printf("write %d %d\n",ret,total);
 	}