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>