Input: serio_raw - return proper result when serio_raw_write fails

If serio_raw_write was always returning number of bytes successfully
sent to serio port and never signalled error condition to the caller.
Change it so that for completely failed transfers appropriate error
code returned to the caller (partially successful writes still return
number of bytes transferred).
Reported-by: NChe-liang Chiou <clchiou@chromium.org>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

drivers/input/serio/serio_raw.c

@@ -224,7 +224,7 @@ static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
 
 out:
 	mutex_unlock(&serio_raw_mutex);
-	return written;
+	return written ?: retval;
 }
 
 static unsigned int serio_raw_poll(struct file *file, poll_table *wait)