net/mlx4_core: Fix obscure mlx4_cmd_box parameter in QUERY_DEV_CAP

The "!mlx4_is_slave" is totally confusing.  Fix with
constant MLX4_CMD_NATIVE, which is the intended behavior.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>