r8169: Avoid pointer aliasing

Read MAC address 32-bit at a time and manually extract the individual
bytes. This avoids pointer aliasing and gives the compiler a better
chance of optimizing the operation.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>