net/mlx4: fixing sparse warnings when copying mac, address to gid entry

The mac should be written as __be64 the gid. The warning was because
we changed the mac parameter, which is u64, by writing result of cpu_to_be64
into it. Fixing by using new variable of type __be64.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>