RDMA/core: Avoid freeing netdevs in disable_device()

So we can use the disable_device() helper while changing the net namespace
of the rdma device in a subsequent patch, move free_netdevs() out of it.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>