USB: XHCI: fix memory leak of URB-private data

This patch (as1640) fixes a memory leak in xhci-hcd.  The urb_priv
data structure isn't always deallocated in the handle_tx_event()
routine for non-control transfers.  The patch adds a kfree() call so
that all paths end up freeing the memory properly.

This patch should be backported to kernels as old as 2.6.36, that
contain the commit 8e51adcc "USB: xHCI:
Introduce urb_priv structure"

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-and-tested-by: Martin Mokrejs <mmokrejs@fold.natur.cuni.cz>
CC: <stable@vger.kernel.org>