xhci: Regression fix for 3.13.

Hi Greg,

Here's one bug fix for 3.13.

usb-net added support for bulk scatter-gather in 3.12, and it triggered a
bug in the xHCI driver.  This bug causes xHCI hosts to send an unexpected
short transfer, which will cause the USB ethernet device to stop sending
packets.

The patch is marked for the 3.12 stable kernel.  It's a long standing bug,
but the usb-net drivers are the first to trigger it.  The only other
driver that does bulk scatter-gather (usb-storage) will not trigger this
bug.

I'm not sure what the effect of the no-op TRBs will be on various xHCI
host controllers, so I would only like to be conservative and only queue
it for 3.13 and 3.12 stable.

Please queue this for 3.13.

Sarah Sharp
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