PCI: pciehp: create files only for existing capabilities

Current pciehp driver creates 'attention' and 'latch' files even if
the controller doesn't support them. In this case, the contents of
those files are meaningless and unpredictable. Those files should be
created only if the controller has the corresponding capabilities.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>