net/mlx5e: Use PTR_ERR_OR_ZERO() to simplify code

Use the standard error pointer macro to shorten the code and simplify.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>