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/sb1250-duart.c b/drivers/serial/sb1250-duart.c
index fa5f303..a2f2b32 100644
--- a/drivers/serial/sb1250-duart.c
+++ b/drivers/serial/sb1250-duart.c
@@ -440,7 +440,7 @@
 
 	if (delta & ((M_DUART_IN_PIN2_VAL | M_DUART_IN_PIN0_VAL) <<
 		     S_DUART_IN_PIN_CHNG))
-		wake_up_interruptible(&uport->state->delta_msr_wait);
+		wake_up_interruptible(&uport->state->port.delta_msr_wait);
 }
 
 static irqreturn_t sbd_interrupt(int irq, void *dev_id)