tg3: Fix tg3_skb_error_unmap()

This function attempts to free one fragment beyond the number of
fragments that were actually mapped.  This patch brings back the limit
to the correct spot.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>