qeth: Fix missing pointer update

qeth_hdr_chk_and_bounce() can possibly shift the skb->data
pointer. However, the existing code didn't update the hdr pointer,
which should point to skb->data, accordingly.
Symptoms of this issue are sporadic recoveries.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>