serial: move delta_msr_wait into the tty_port

This is used by various drivers not just serial and can be extracted
as commonality

Signed-off-by: Alan Cox <alan@linux.intel.com>
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 9fdc3d8..0daa8a7 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -203,6 +203,7 @@
 	int			count;		/* Usage count */
 	wait_queue_head_t	open_wait;	/* Open waiters */
 	wait_queue_head_t	close_wait;	/* Close waiters */
+	wait_queue_head_t	delta_msr_wait;	/* Modem status change */
 	unsigned long		flags;		/* TTY flags ASY_*/
 	struct mutex		mutex;		/* Locking */
 	unsigned char		*xmit_buf;	/* Optional buffer */