net: dsa: read mac address from DT for slave device

Before creating a slave netdevice, get the mac address from DTS and
apply in case it is valid.

Signed-off-by: Xiaofei Shen <xiaofeis@codeaurora.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>