net/mlx4_core: Dynamically allocate structs at mlx4_slave_cap

In order to avoid temporary large structs on the stack,
allocate them dynamically.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Tal Alon <talal@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>