PCI/ASPM: Add comment about L1 substate latency

Since the exit latencies for L1 substates are not advertised by a device,
it is not clear in spec how to do a L1 substate exit latency check.  We
assume that the L1 exit latencies advertised by a device include L1
substate latencies (and hence do not do any check).  If that is not true,
we should do some sort of check here.

(I'm not clear about what that check should like currently. I'd be glad to
take up any suggestions).

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