igb: Use PCI Express Capability accessors

Use PCI Express Capability access functions to simplify igb driver.

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