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/drivers/serial/zs.c b/drivers/serial/zs.c
index b9c9fb9..1a7fd3e 100644
--- a/drivers/serial/zs.c
+++ b/drivers/serial/zs.c
@@ -686,7 +686,7 @@
 			uport->icount.rng++;
 
 		if (delta)
-			wake_up_interruptible(&uport->state->delta_msr_wait);
+			wake_up_interruptible(&uport->state->port.delta_msr_wait);
 
 		spin_lock(&scc->zlock);
 	}