cxgb3: Use PCI Express Capability accessors

Use PCI Express Capability access functions to simplify cxgb3 driver.

[bhelgaas: split cxgb3 and cxgb4 into separate patches]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>