isdn: hisax: Use l2headersize() instead of dup (and buggy) func.

There was a bug in my commit c978e7bb
("hisax: Fix unchecked alloc_skb() return.")

One of the l2->flag checks is wrong.

Even worse it turns out I'm duplicating an existing function,
so use that instead.
Reported-by: Milton Miller <miltonm@bga.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/hisax/isdnl2.c

@@ -1243,13 +1243,6 @@ l2_st7_tout_203(struct FsmInst *fi, int event, void *arg)
 	st->l2.rc = 0;
 }
 
-static int l2_hdr_space_needed(struct Layer2 *l2)
-{
-	int len = test_bit(FLG_LAPD, &l2->flag) ? 2 : 1;
-
-	return len + (test_bit(FLG_LAPD, &l2->flag) ? 2 : 1);
-}
-
 static void
 l2_pull_iqueue(struct FsmInst *fi, int event, void *arg)
 {
@@ -1268,7 +1261,7 @@ l2_pull_iqueue(struct FsmInst *fi, int event, void *arg)
 	if (!skb)
 		return;
 
-	hdr_space_needed = l2_hdr_space_needed(l2);
+	hdr_space_needed = l2headersize(l2, 0);
 	if (hdr_space_needed > skb_headroom(skb)) {
 		struct sk_buff *orig_skb = skb;