r8152: fix r8152_csum_workaround function

The transport offset of the IPv4 packet should be fixed and wouldn't
be out of the hw limitation, so the r8152_csum_workaround() should
be used for IPv6 packets.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>