net: phy: constify netdev->dev_addr references

netdev->dev_addr will become a const soon(ish),
constify the local variables referring to it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>