net/mlx4_en: Fix mac_hash database inconsistency

Using a local copy of dev_addr in mlx4_en_set_mac() to prevent dev_addr
from being modified during error flow or when dev_addr is modified in
another context (which is another problem that is being discussed over
the mailing list [1]).
Also fixing bad naming of priv->prev_mac into priv->current_mac.

[1] - http://patchwork.ozlabs.org/patch/351489/



Reviewed-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>