tty: Fix hvc return

HVC returns a size of -1 bytes for the write room in some cases.
This is bogus and not handled by the tty layer at all.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 7f80f15..4a65299 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -558,7 +558,7 @@
 	struct hvc_struct *hp = tty->driver_data;
 
 	if (!hp)
-		return -1;
+		return 0;
 
 	return hp->outbuf_size - hp->n_outbuf;
 }