Use PCI Express Capability access functions to simplify igb driver.

[bhelgaas: split e1000e and igb into separate patches]
Signed-off-by: NJiang Liu <jiang.liu@huawei.com>
Signed-off-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>