net/mlx5: Use flow counter pointer as input to the query function

This allows to un-expose the details of struct mlx5_fc and keep it
internal to the core driver.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>