IB/IPoIB: Fix error code in ipoib_add_port()

We accidentally don't see the error code on some of these error paths.
It means we return ERR_PTR(0) which is NULL and it results in a NULL
dereference in the caller.

This bug dates to pre-git days.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>