net/mlx5e: Return error when trying to insert existing flower filter

With unlocked TC it is possible to have spurious deletes and inserts of
same filter. TC layer needs drivers to always return error when flow
insertion failed in order to correctly calculate "in_hw_count" for each
filter. Fix mlx5e_configure_flower() to return -EEXIST when TC tries to
insert a filter that is already provisioned to the driver.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>