net: bcmgenet: fix bcmgenet_put_tx_csum()

bcmgenet_put_tx_csum() needs to return skb pointer back to the caller
because it reallocates a new one in case of lack of skb headroom.

Signed-off-by: Petri Gynther <pgynther@google.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>