xhci: Don't show incorrect WARN message about events for empty rings

xHC can generate two events for a short transfer if the short TRB and
last TRB in the TD are not the same TRB.

The driver will handle the TD after the first short event, and remove
it from its internal list. Driver then incorrectly prints a warning
for the second event:

"WARN Event TRB for slot x ep y with no TDs queued"

Fix this by not printing a warning if we get a event on a empty list
if the previous event was a short event.

Cc: <stable@vger.kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>