sysfs: Initialised pci bus legacy_mem field before use

PPC64 is failing to boot the latest mmotm due to an uninitialised pointer in
pci_create_legacy_files(). The surprise is that machines boot at all and it
would appear to affect current mainline as well.  This patch fixes the problem.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>