net/mlx5: Refactor HCA capability set flow

Reduce the amount of kzalloc/kfree cycles by allocating
command structure in the parent function and leverage the
knowledge that set_caps() is called for HCA capabilities
only with specific HW structure as parameter to calculate
mailbox size.

Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>