USB: xhci: Fix bug memory free after failed initialization.

If the xHCI driver fails during the memory initialization, xhci->ir_set
may not be a valid pointer.  Check that it points to valid DMA'able memory
before writing to that address during the memory freeing process.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci-mem.c

@@ -802,9 +802,11 @@ void xhci_mem_cleanup(struct xhci_hcd *xhci)
 	int i;
 
 	/* Free the Event Ring Segment Table and the actual Event Ring */
-	xhci_writel(xhci, 0, &xhci->ir_set->erst_size);
-	xhci_write_64(xhci, 0, &xhci->ir_set->erst_base);
-	xhci_write_64(xhci, 0, &xhci->ir_set->erst_dequeue);
+	if (xhci->ir_set) {
+		xhci_writel(xhci, 0, &xhci->ir_set->erst_size);
+		xhci_write_64(xhci, 0, &xhci->ir_set->erst_base);
+		xhci_write_64(xhci, 0, &xhci->ir_set->erst_dequeue);
+	}
 	size = sizeof(struct xhci_erst_entry)*(xhci->erst.num_entries);
 	if (xhci->erst.entries)
 		pci_free_consistent(pdev, size,