eth: fwnode: remove the addr len from mac helpers

All callers pass in ETH_ALEN and the function itself
will return -EINVAL for any other address length.
Just assume it's ETH_ALEN like all other mac address
helpers (nvm, of, platform).

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