net/mlx4_core: Fix cast warning in fw.c

Fix the following SPARSE warning, in MLX4_GET() macro:
drivers/net/ethernet/mellanox/mlx4/fw.c:233:9: warning: cast to restricted __be64

Fixes: 17d5ceb6 ("net/mlx4_core: Fix unaligned accesses")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>