USB: xhci: Clean up xhci_irq() function.

Drop spinlock in xhci_irq() error path.
This fixes the issue reported by Oliver Neukum on this thread:
	http://marc.info/?l=linux-usb&m=124090924401444&w=2

Remove unnecessary register read reported by Viral Mehta:
	http://marc.info/?l=linux-usb&m=124091326007398&w=2

Reported-by: Oliver Neukum <oliver@neukum.org>
Reported-by: Viral Mehta <viral.mehta@einfochips.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci-hcd.c

@@ -276,11 +276,11 @@ irqreturn_t xhci_irq(struct usb_hcd *hcd)
 		return IRQ_NONE;
 	}
 
-	temp = xhci_readl(xhci, &xhci->op_regs->status);
 	if (temp & STS_FATAL) {
 		xhci_warn(xhci, "WARNING: Host System Error\n");
 		xhci_halt(xhci);
 		xhci_to_hcd(xhci)->state = HC_STATE_HALT;
+		spin_unlock(&xhci->lock);
 		return -ESHUTDOWN;
 	}