network: Modify ifup and ifdown while setting IP address
network: Modify ifup and ifdown while setting IP address

While netdev was requested to change IP address it turns down the interface
temporarily for preventing a address conflict.
However, it turns down device functionalities which takes time now.
So by calling netdev_soft_ifdown, netdev won't turn down it. so it can
reduce time.
Furthermore calling netif_set_* is not thread-safe. API
family(netifapi_netif_set*) are more safe.

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