net/mlx5e: Avoid duplicated code for tc offloads add/del fdb rule

The code for adding/deleting fdb flow is repeated when
user-space does flow add/del and when we add/del from
the neigh update path - unify them to avoid the duplication.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>