PCI: Use u8, not int, for PM capability offset

The Power Management Capability (PCI_CAP_ID_PM == 0x01) is defined by PCI
and must appear in the 256-byte PCI Configuration Space from 0-0xff.  It
cannot be in the PCIe Extended Configuration space from 0x100-0xfff, so
we only need a u8 to hold its offset.

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