net: remove single-byte netdev->dev_addr writes

Make the drivers which use single-byte netdev addresses
(netdev->addr_len == 1) use the appropriate address setting
helpers.

arcnet copies from int variables and io reads a lot, so
add a helper for arcnet drivers to use.

Similar helper could be reused for phonet and appletalk
but there isn't any good central location where we could
put it, and netdevice.h is already very crowded.

Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com> # for HSI
Link: https://lore.kernel.org/r/20211012142757.4124842-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>