net: unexport dev_addr_init() & dev_addr_flush()

There are no module callers in-tree and it's hard to justify
why anyone would init or flush addresses of a netdev (note
the flush is more of a destructor, it frees netdev->dev_addr).

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>