net: create netdev->dev_addr assignment helpers

[ Upstream commit 48eab831ae8b9f7002a533fa4235eed63ea1f1a3 ]

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>
Stable-dep-of: 391af06a02e7 ("wifi: wl3501_cs: Fix an error handling path in wl3501_probe()")
Signed-off-by: Sasha Levin <sashal@kernel.org>