Rationalize fasync return values

Most fasync implementations do something like:

     return fasync_helper(...);

But fasync_helper() will return a positive value at times - a feature used
in at least one place.  Thus, a number of other drivers do:

     err = fasync_helper(...);
     if (err < 0)
             return err;
     return 0;

In the interests of consistency and more concise code, it makes sense to
map positive return values onto zero where ->fasync() is called.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

drivers/hid/usbhid/hiddev.c

@@ -227,12 +227,9 @@ void hiddev_report_event(struct hid_device *hid, struct hid_report *report)
  */
 static int hiddev_fasync(int fd, struct file *file, int on)
 {
-	int retval;
 	struct hiddev_list *list = file->private_data;
 
-	retval = fasync_helper(fd, file, on, &list->fasync);
-
-	return retval < 0 ? retval : 0;
+	return fasync_helper(fd, file, on, &list->fasync);
 }