usb: xhci: fix wild pointers in xhci_mem_cleanup

This patch fixes some wild pointers produced by xhci_mem_cleanup.
These wild pointers will cause system crash if xhci_mem_cleanup()
is called twice.

Reported-and-tested-by: Pengcheng Li <lpc.li@hisilicon.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>