PCI PM: Check if the state has been saved before trying to restore it

Check if the standard configuration registers of a PCI device have
been saved during suspend before trying to restore them during
resume.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-By: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>