octeon_ep: fix error return code in octep_probe()

If register_netdev() fails , it should return error
code in octep_probe().

Fixes: 862cd659 ("octeon_ep: Add driver framework and device initialization")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>