x86: remove bogus 'pci=usepirqmask' suggestion when no irq is defined

This was harmless, but for the case of a device that had no irq
pre-defined we would incorrectly suggest that "usepirqmask" might make a
difference.  It never would, and the message was just confusing people.

Reported in the dmesg of Etienne Lorrain.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/pci/irq.c

@@ -846,7 +846,7 @@ static int pcibios_lookup_irq(struct pci_dev *dev, int assign)
 	 * reported by the device if possible.
 	 */
 	newirq = dev->irq;
-	if (!((1 << newirq) & mask)) {
+	if (newirq && !((1 << newirq) & mask)) {
 		if ( pci_probe & PCI_USE_PIRQ_MASK) newirq = 0;
 		else printk(KERN_WARNING "PCI: IRQ %i for device %s doesn't match PIRQ mask - try pci=usepirqmask\n", newirq, pci_name(dev));
 	}