drivers/net: eliminate irq handler impossible checks, needless casts

- Eliminate check for irq handler 'dev_id==NULL' where the
  condition never occurs.

- Eliminate needless casts to/from void*

Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/drivers/net/skfp/skfddi.c b/drivers/net/skfp/skfddi.c
index 06ea262..9733a11 100644
--- a/drivers/net/skfp/skfddi.c
+++ b/drivers/net/skfp/skfddi.c
@@ -616,15 +616,10 @@
 
 irqreturn_t skfp_interrupt(int irq, void *dev_id)
 {
-	struct net_device *dev = (struct net_device *) dev_id;
+	struct net_device *dev = dev_id;
 	struct s_smc *smc;	/* private board structure pointer */
 	skfddi_priv *bp;
 
-	if (dev == NULL) {
-		printk("%s: irq %d for unknown device\n", dev->name, irq);
-		return IRQ_NONE;
-	}
-
 	smc = netdev_priv(dev);
 	bp = &smc->os;