BUG_ON() Conversion in drivers/isdn

this changes if() BUG(); constructs to BUG_ON() which is
cleaner, contains unlikely() and can better optimized away.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/drivers/isdn/hisax/st5481_d.c b/drivers/isdn/hisax/st5481_d.c
index 941f702..493dc94 100644
--- a/drivers/isdn/hisax/st5481_d.c
+++ b/drivers/isdn/hisax/st5481_d.c
@@ -596,9 +596,7 @@
 		break;
 	case PH_DATA | REQUEST:
 		DBG(2, "PH_DATA REQUEST len %d", skb->len);
-		if (adapter->d_out.tx_skb)
-			BUG();
-
+		BUG_ON(adapter->d_out.tx_skb);
 		adapter->d_out.tx_skb = skb;
 		FsmEvent(&adapter->d_out.fsm, EV_DOUT_START_XMIT, NULL);
 		break;