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/atmel_serial.c b/drivers/serial/atmel_serial.c
index 963e3c1..3551c5c 100644
--- a/drivers/serial/atmel_serial.c
+++ b/drivers/serial/atmel_serial.c
@@ -776,7 +776,7 @@
 		if (status_change & ATMEL_US_CTS)
 			uart_handle_cts_change(port, !(status & ATMEL_US_CTS));
 
-		wake_up_interruptible(&port->state->delta_msr_wait);
+		wake_up_interruptible(&port->state->port.delta_msr_wait);
 
 		atmel_port->irq_status_prev = status;
 	}