nfp: fix error return code in nfp_net_netdev_open()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 73725d9d ("nfp: allocate ring SW structs dynamically")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>