net/mlx5e: add missing error code in error path

Add missing error code when mlx5e_macsec_fs_add_rule() or
mlx5e_macsec_fs_init() fails. mlx5e_macsec_fs_init() don't
return ERR_PTR(), so replace IS_ERR_OR_NULL() check with
NULL pointer check.

Fixes: e467b283 ("net/mlx5e: Add MACsec TX steering rules")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://lore.kernel.org/r/20220914140100.3795545-1-yangyingliang@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>