PCI: pciehp: Drop pciehp_readw()/pciehp_writew() wrappers

The pciehp_readw() and pciehp_writew() wrappers only look up the pci_dev
and call the PCIe Capability accessors, so we can make things a little
more straightforward by just using the PCIe Capability accessors directly.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>