net: connection: Make it possible to update local address

Allow to update the local address on a registered connection when
rebinding.

This is needed for packet sockets, as by default packet socket
will be bound to "any" interface (interface index 0), and interface
index is part of the local address registered for packet socket.
In order to be able to explicitly bind to a specific interface later, it
needs to be possible to update the local address registered for the
connection, as we need to update the interface index, which is used
by net_conn_packet_input() for packet filtering.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>