net/mlx5e: Change return type of tc add flow functions

Refactor the flow add utility functions to return err code instead of rule
pointers. This will allow for simpler logic when one tc rule is
duplicated to two HW rules in downstream patches.

Signed-off-by: Rabie Loulou <rabiel@mellanox.com>
Signed-off-by: Shahar Klein <shahark@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>