lspci: Decode only supported ASPM exit latencies

lspci: Decode only supported ASPM exit latencies

Per PCIe spec r3.1, sec 7.8.6, the L0s Exit Latency is only valid when L0s
is supported, and similarly the L1 Exit Latency is only valid when L1 is
supported.

Only decode the L0s and L1 Exit Latencies if they are defined.

For example, on a device that supports L1 but not L0s, the difference in
the "lspci -vv" output looks like this:

-   LnkCap: Port #1, Speed 8GT/s, Width x1, ASPM L1, Exit Latency L0s <1us, L1 <16us
+   LnkCap: Port #1, Speed 8GT/s, Width x1, ASPM L1, Exit Latency L1 <16us

Correct the comments on the PCI_EXP_LNKCAP_L0S and PCI_EXP_LNKCAP_L1
definitions.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>