net/mlx5e: Extract code that queues neigh update work into function

As a preparation for following refactoring that removes rtnl lock
dependency from neigh hash entry handlers, extract code that enqueues neigh
update work into standalone function. This commit doesn't change
functionality.

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