tg3: Fix 4k skb error recovery path

On the error recovery resource unwind path, it is possible for the
driver to attempt to unmap a fragment that hadn't been mapped.  This
patch fixes the problem by correcting the "last" parameter supplied.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>