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/sunlance.c b/drivers/net/sunlance.c
index 9207e19..5b00d79 100644
--- a/drivers/net/sunlance.c
+++ b/drivers/net/sunlance.c
@@ -822,7 +822,7 @@
 
 static irqreturn_t lance_interrupt(int irq, void *dev_id)
 {
-	struct net_device *dev = (struct net_device *)dev_id;
+	struct net_device *dev = dev_id;
 	struct lance_private *lp = netdev_priv(dev);
 	int csr0;