ethernet: tg3: remove direct netdev->dev_addr writes

tg3 does various forms of direct writes to netdev->dev_addr.
Use a local buffer. Make sure local buffer is aligned since
eth_platform_get_mac_address() may call ether_addr_copy().

tg3_get_device_address() returns whenever it finds a method
that found a valid address. Instead of modifying all the exit
points pass the buffer from the outside and commit the address
in the caller.

Constify the argument of the set addr helper.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>