USB: Enable Latency Tolerance Messaging (LTM).

USB 3.0 devices may optionally support a new feature called Latency
Tolerance Messaging.  If both the xHCI host controller and the device
support LTM, it should be turned on in order to give the system hardware
a better clue about the latency tolerance values of its PCI devices.

Once a Set Feature request to enable LTM is received, the USB 3.0 device
will begin to send LTM updates as its buffers fill or empty, and it can
tolerate more or less latency.

The USB 3.0 spec, section C.4.2 says that LTM should be disabled just
before the device is placed into suspend.  Then the device will send an
updated LTM notification, so that the system doesn't think it should
remain in an active state in order to satisfy the latency requirements
of the suspended device.

The Set and Clear Feature LTM enable command can only be sent to a
configured device.  The device will respond with an error if that
command is sent while it is in the Default or Addressed state.  Make
sure ...