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/3c509.c b/drivers/net/3c509.c
index 7ad0a54..f791bf0 100644
--- a/drivers/net/3c509.c
+++ b/drivers/net/3c509.c
@@ -912,16 +912,11 @@
 static irqreturn_t
 el3_interrupt(int irq, void *dev_id)
 {
-	struct net_device *dev = (struct net_device *)dev_id;
+	struct net_device *dev = dev_id;
 	struct el3_private *lp;
 	int ioaddr, status;
 	int i = max_interrupt_work;
 
-	if (dev == NULL) {
-		printk ("el3_interrupt(): irq %d for unknown device.\n", irq);
-		return IRQ_NONE;
-	}
-
 	lp = netdev_priv(dev);
 	spin_lock(&lp->lock);