Compare the correct variable against NULL.

Reviewed by:	scottl
Found with:	Coverity Prevent(tm)
CID:		821
MFC after:	2 weeks

sys/dev/iir/iir_pci.c

@@ -202,7 +202,7 @@ iir_pci_attach(device_t dev)
     rid = 0;
     irq = bus_alloc_resource_any(dev, SYS_RES_IRQ, &rid,
                                  RF_ACTIVE | RF_SHAREABLE);
-    if (io == NULL) {
+    if (irq == NULL) {
         device_printf(dev, "can't find IRQ value\n");
         error = ENOMEM;
         goto err;