tty: rework break handling

Some hardware needs to do break handling itself and may have partial
support only. Make break_ctl return an error code. Add a tty driver flag
so you can indicate driver hardware side break support.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 95ae637..0725b18 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -732,13 +732,16 @@
 	tasklet_schedule(&acm->urb_task);
 }
 
-static void acm_tty_break_ctl(struct tty_struct *tty, int state)
+static int acm_tty_break_ctl(struct tty_struct *tty, int state)
 {
 	struct acm *acm = tty->driver_data;
+	int retval;
 	if (!ACM_READY(acm))
-		return;
-	if (acm_send_break(acm, state ? 0xffff : 0))
+		return -EINVAL;
+	retval = acm_send_break(acm, state ? 0xffff : 0);
+	if (retval < 0)
 		dbg("send break failed");
+	return retval;
 }
 
 static int acm_tty_tiocmget(struct tty_struct *tty, struct file *file)