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>