tg3: Generalize tg3_skb_error_unmap()

In the following patches, unmapping skb fragments will get just as
complicated as mapping them.  This patch generalizes
tg3_skb_error_unmap() and makes it the one-stop-shop for skb unmapping.

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