PM: Make suspend_device() static

suspend_device() can become static.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Len Brown <len.brown@intel.com>