PCI: Fix kerneldoc for pci_disable_link_state()

Fix kerneldoc for pci_disable_link_state().

[bhelgaas: expand comment, fix typos]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>