mtip32xx: prefer pcie_capability_read_word()

Replace pci_read_config_word() with pcie_capability_read_word().

pcie_capability_read_word() takes care of a few special cases when reading
the PCIe capability.  See 8c0d3a02 ("PCI: Add accessors for PCI Express
Capability").

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>