Separate struct pcib_softc slot state away from slot flags, and
make state transitions more discrete adding both an attached and
detached state
When PDS is cleared, only proceed to DETACHED state from
DETACHING to prevent a bogus PDS 0 on device insertion from
stopping us from attaching if the link is present
In pcib_hotplug_inserted, allow either PDS in the slot state or
DL_ACTIVE in the link state to indicate presence of a device so
that a bogus PDS 0 does not prevent attaching on an active link
In pcib_pcie_dll_timeout, only transition to the DETACHING state
if a hotplug event had put the slot into a DETACH_PENDING state
so that the state machine does not get stuck in DETACHING after
removing a device and the bogus PDS 1 causes the wait for the
DL_ACTIVE change to time out