net/mlx5: Improve argument name for add flow API

The last argument to mlx5_add_flow_rules passes the number of
destinations in the struct pointed to by the dest arg. Change the name
to better reflect this fact.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>