IB/ipoib: Add child to parent list only if device initialized

Despite failure in ipoib_dev_init() we continue with initialization flow
and creation of child device. It causes to the situation where this child
device is added too early to parent device list.

Change the logic, so in case of failure we properly return error from
ipoib_dev_init() and add child only in success path.

Fixes: eaeb3984 ("IB/ipoib: Move init code to ndo_init")
Signed-off-by: Valentine Fatiev <valentinef@mellanox.com>
Reviewed-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>