PCI: Use pdev->pm_cap instead of pci_find_capability(..,PCI_CAP_ID_PM)

PCI PM cap register offset has been saved in pci_pm_init(),
so we can use pdev->pm_cap instead of using pci_find_capability(..)
here.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>