r8169: avoid potentially misaligned access when getting mac address

Interpreting a member of an u16 array as u32 may result in a misaligned
access. Also it's not really intuitive to define a mac address variable
as array of three u16 words. Therefore use an array of six bytes that
is properly aligned for 32 bit access.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>