r8152: fix accessing unset transport header

A warning is triggered by commit 66e4c8d9 ("net: warn if transport
header was not set"). The warning is harmless, because the value from
skb_transport_offset() is only used for skb_is_gso() is true or the
skb->ip_summed is equal to CHECKSUM_PARTIAL.

Fixes: 66e4c8d9 ("net: warn if transport header was not set")
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>