[SCSI] pm8001: use pdev->pm_cap instead of pci_find_capability(..,PCI_CAP_ID_PM)

Pci core has been saved pm cap register offset by pdev->pm_cap in
pci_pm_init() in init path. So we can use pdev->pm_cap instead of using
pci_find_capability(pdev, PCI_CAP_ID_PM) for better performance and simplified
code.

Tested-by: Lindar Liu <lindar_liu@usish.com>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Acked-by: Lindar Liu <lindar_liu@usish.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>