Intel xhci: Limit number of active endpoints to 64.

The Panther Point chipset has an xHCI host controller that has a limit to
the number of active endpoints it can handle.  Ideally, it would signal
that it can't handle anymore endpoints by returning a Resource Error for
the Configure Endpoint command, but they don't.  Instead it needs software
to keep track of the number of active endpoints, across configure endpoint
commands, reset device commands, disable slot commands, and address device
commands.

Add a new endpoint context counter, xhci_hcd->num_active_eps, and use it
to track the number of endpoints the xHC has active.  This gets a little
tricky, because commands to change the number of active endpoints can
fail.  This patch adds a new xHCI quirk for these Intel hosts, and the new
code should not have any effect on other xHCI host controllers.

Fail a new device allocation if we don't have room for the new default
control endpoint.  Use the endpoint ring pointers to determine what
endpoints were active before a Reset Device command or a Disable Slot
command, and drop those once the command completes.

Fail a configure endpoint command if it would add too many new endpoints.
We have to be a bit over zealous here, and only count the number of new
endpoints to be added, without subtracting the number of dropped
endpoints.  That's because a second configure endpoint command for a
different device could sneak in before we know if the first command is
completed.  If the first command dropped resources, the host controller
fails the command for some reason, and we're nearing the limit of
endpoints, we could end up oversubscribing the host.

To fix this race condition, when evaluating whether a configure endpoint
command will fix in our bandwidth budget, only add the new endpoints to
xhci->num_active_eps, and don't subtract the dropped endpoints.  Ignore
changed endpoints (ones that are dropped and then re-added), as that
shouldn't effect the host's endpoint resources.  When the configure
endpoint command completes, subtract off the dropped endpoints.

This may mean some configuration changes may temporarily fail, but it's
always better to under-subscribe than over-subscribe resources.

(Originally my plan had been to push the resource allocation down into the
ring allocation functions.  However, that would cause us to allocate
unnecessary resources when endpoints were changed, because the xHCI driver
allocates a new ring for the changed endpoint, and only deletes the old
ring once the Configure Endpoint command succeeds.  A further complication
would have been dealing with the per-device endpoint ring cache.)

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

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