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

There was a bug in my commit c978e7bb77dfd2cd3d1f547fa4e395cfe47f02b2
("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>