net/mlx5: Remove redundant NULL initializations

Neighbour initializations to NULL are not necessary as the pointers are
not used if an error is returned, and if success returned, pointers are
initialized.

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