net: create netdev->dev_addr assignment helpers

Recent work on converting address list to a tree made it obvious
we need an abstraction around writing netdev->dev_addr. Without
such abstraction updating the main device address is invisible
to the core.

Introduce a number of helpers which for now just wrap memcpy()
but in the future can make necessary changes to the address
tree.

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