IB/ipoib: Move init code to ndo_init

Now that we have a proper ndo_uninit, move code that naturally pairs
with the ndo_uninit into ndo_init. This allows the netdev core to natually
handle ordering.

This fixes the situation where register_netdev can fail before calling
ndo_init, in which case it wouldn't call ndo_uninit either.

Also move a bunch of duplicated init code that is shared between child
and parent for clarity. Now the child and parent register functions look
very similar.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>