PCI ASPM: cleanup misc in struct pcie_link_state

Cleanup for some fields in pcie_link_state.

- Add comments.
- make "downstream_has_switch" field 1-bit.

Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>