net/mlx5: remove many unnecessary NULL values

There are many pointers assigned first, which need not to be
initialized, so remove the NULL assignments.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>