[Serial] Don't miss modem status changes

Reading the MSR register on 8250-compatible UARTs results in any
modem status interrupts being cleared.  To avoid missing any
status changes, arrange for get_mctrl() to read the current
status via check_modem_status(), which will process any pending
state changes for us.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/serial/8250.c

@@ -1255,15 +1255,11 @@ static _INLINE_ void transmit_chars(struct uart_8250_port *up)
 		__stop_tx(up);
 }
 
-static _INLINE_ void check_modem_status(struct uart_8250_port *up)
+static unsigned int check_modem_status(struct uart_8250_port *up)
 {
-	int status;
-
-	status = serial_in(up, UART_MSR);
-
-	if ((status & UART_MSR_ANY_DELTA) == 0)
-		return;
+	unsigned int status = serial_in(up, UART_MSR);
 
+	if (status & UART_MSR_ANY_DELTA && up->ier & UART_IER_MSI) {
 		if (status & UART_MSR_TERI)
 			up->port.icount.rng++;
 		if (status & UART_MSR_DDSR)
@@ -1274,6 +1270,9 @@ static _INLINE_ void check_modem_status(struct uart_8250_port *up)
 			uart_handle_cts_change(&up->port, status & UART_MSR_CTS);
 
 		wake_up_interruptible(&up->port.info->delta_msr_wait);
+	}
+
+	return status;
 }
 
 /*
@@ -1454,10 +1453,10 @@ static unsigned int serial8250_tx_empty(struct uart_port *port)
 static unsigned int serial8250_get_mctrl(struct uart_port *port)
 {
 	struct uart_8250_port *up = (struct uart_8250_port *)port;
-	unsigned char status;
+	unsigned int status;
 	unsigned int ret;
 
-	status = serial_in(up, UART_MSR);
+	status = check_modem_status(up);
 
 	ret = 0;
 	if (status & UART_MSR_DCD)