net: bcmgenet: return NULL instead of plain integer

Fixes the following sparse warning:

drivers/net/ethernet/broadcom/genet/bcmgenet.c:1351:16: warning:
 Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>