serial: 8250_dw: fix compilation warning when !CONFIG_PM_SLEEP

CONFIG_PM will be set if either or both CONFIG_PM_SLEEP and
CONFIG_PM_RUNTIME is set. Compiling the driver with !CONFIG_PM_SLEEP causes
following compilation warnings:

drivers/tty/serial/8250/8250_dw.c:404:12: warning: ‘dw8250_suspend’ defined but not used [-Wunused-function]
drivers/tty/serial/8250/8250_dw.c:413:12: warning: ‘dw8250_resume’ defined but not used [-Wunused-function]

Fix this by using CONFIG_PM_SLEEP instead.
Signed-off-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dw.c

@@ -391,7 +391,7 @@ static int dw8250_remove(struct platform_device *pdev)
 	return 0;
 }
 
-#ifdef CONFIG_PM
+#ifdef CONFIG_PM_SLEEP
 static int dw8250_suspend(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
@@ -409,7 +409,7 @@ static int dw8250_resume(struct device *dev)
 
 	return 0;
 }
-#endif /* CONFIG_PM */
+#endif /* CONFIG_PM_SLEEP */
 
 #ifdef CONFIG_PM_RUNTIME
 static int dw8250_runtime_suspend(struct device *dev)