IB/ipoib: Fix error return code in ipoib_dev_init()

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

Fixes: 515ed4f3 ("IB/IPoIB: Separate control and data related initializations")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>