PNP: skip UNSET MEM resources as well as DISABLED ones

We don't need to reserve "unset" resources.  Trying to reserve
them results in messages like this, which are ugly but harmless:

    system 00:08: iomem range 0x0-0x0 could not be reserved

Future PNP patches will remove use of IORESOURCE_UNSET, but
we still need it for now.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>