Use PCI Express Capability access functions to simplify myri10ge driver.

[bhelgaas: fix myri10ge_toggle_relaxed() return value]
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>