atmel_serial: Drain console TX shifter before suspending

Funny things may happen if we stop the USART clock before the shifter is
empty. Prevent this from happening by waiting until the shifter is
completely drained before allowing suspend to continue.
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>

drivers/serial/atmel_serial.c

@@ -1454,6 +1454,12 @@ static int atmel_serial_suspend(struct platform_device *pdev,
 	struct uart_port *port = platform_get_drvdata(pdev);
 	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
 
+	if (atmel_is_console_port(port) && console_suspend_enabled) {
+		/* Drain the TX shifter */
+		while (!(UART_GET_CSR(port) & ATMEL_US_TXEMPTY))
+			cpu_relax();
+	}
+
 	if (device_may_wakeup(&pdev->dev)
 	    && !atmel_serial_clk_will_stop())
 		enable_irq_wake(port->irq);