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/saa9730.c b/drivers/net/saa9730.c
index c9efad8..b269513 100644
--- a/drivers/net/saa9730.c
+++ b/drivers/net/saa9730.c
@@ -747,7 +747,7 @@
 
 static irqreturn_t lan_saa9730_interrupt(const int irq, void *dev_id)
 {
-	struct net_device *dev = (struct net_device *) dev_id;
+	struct net_device *dev = dev_id;
 	struct lan_saa9730_private *lp = netdev_priv(dev);
 
 	if (lan_saa9730_debug > 5)