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/char/sonypi.c

@@ -888,12 +888,7 @@ found:
 
 static int sonypi_misc_fasync(int fd, struct file *filp, int on)
 {
-	int retval;
-
-	retval = fasync_helper(fd, filp, on, &sonypi_device.fifo_async);
-	if (retval < 0)
-		return retval;
-	return 0;
+	return fasync_helper(fd, filp, on, &sonypi_device.fifo_async);
 }
 
 static int sonypi_misc_release(struct inode *inode, struct file *file)