Input: serio_raw - really signal HUP upon disconnect

Commit 8c1c10d5706bbb3b41cb4a5578339d67d3eeffc2 attempted to signal
POLLHUP | POLLERR condition when polling disconnected device,
unfortunately it did not do it quite correctly.

Reported-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/drivers/input/serio/serio_raw.c b/drivers/input/serio/serio_raw.c
index ca78a89..f6c83e9 100644
--- a/drivers/input/serio/serio_raw.c
+++ b/drivers/input/serio/serio_raw.c
@@ -237,9 +237,9 @@
 
 	mask = serio_raw->dead ? POLLHUP | POLLERR : POLLOUT | POLLWRNORM;
 	if (serio_raw->head != serio_raw->tail)
-		return POLLIN | POLLRDNORM;
+		mask |= POLLIN | POLLRDNORM;
 
-	return 0;
+	return mask;
 }
 
 static const struct file_operations serio_raw_fops = {