drivers/usb/serial/sierra.c: fix CONFIG_PM=n build

drivers/usb/serial/sierra.c: In function 'sierra_suspend':
drivers/usb/serial/sierra.c:936: error: 'struct usb_device' has no member named 'auto_pm'

Repairs

commit e6929a90
Author: Oliver Neukum <oliver@neukum.org>
Date:   Fri Sep 4 23:19:53 2009 +0200

    USB: support for autosuspend in sierra while online

Cc: Greg KH <greg@kroah.com>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/usb/serial/sierra.c

@@ -912,6 +912,7 @@ static void sierra_release(struct usb_serial *serial)
 	}
 }
 
+#ifdef CONFIG_PM
 static void stop_read_write_urbs(struct usb_serial *serial)
 {
 	int i, j;
@@ -988,6 +989,10 @@ static int sierra_resume(struct usb_serial *serial)
 
 	return ec ? -EIO : 0;
 }
+#else
+#define sierra_suspend NULL
+#define sierra_resume NULL
+#endif
 
 static struct usb_serial_driver sierra_device = {
 	.driver = {