usb: xhci: Fix OOPS in xhci error handling code

The xhci driver will OOPS on resume from S2/S3 if dma_alloc_coherent()
is out of memory. This is a result of two things:
1. xhci_mem_cleanup() in xhci-mem.c free's xhci->lpm_command if
it's not NULL, but doesn't set it to NULL after the free.
2. xhci_mem_cleanup() is called twice on resume, once for normal
restart and once from xhci_mem_init() if dma_alloc_coherent() fails,
resulting in a free of xhci->lpm_command that has already been freed.
The fix is to set xhci->lpm_command to NULL after freeing it.

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