net: bonding: fix error return code of bond_neigh_init()

When slave is NULL or slave_ops->ndo_neigh_setup is NULL, no error
return code of bond_neigh_init() is assigned.
To fix this bug, ret is assigned with -EINVAL in these cases.

Fixes: 9e99bfef ("bonding: fix bond_neigh_init()")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>