net: microchip: sparx5: Fix potential null-ptr-deref in sparx_stats_init() and sparx5_start()

sparx_stats_init() calls create_singlethread_workqueue() and not
checked the ret value, which may return NULL. And a null-ptr-deref may
happen:

sparx_stats_init()
    create_singlethread_workqueue() # failed, sparx5->stats_queue is NULL
    queue_delayed_work()
        queue_delayed_work_on()
            __queue_delayed_work()  # warning here, but continue
                __queue_work()      # access wq->flags, null-ptr-deref

Check the ret value and return -ENOMEM if it is NULL. So as
sparx5_start().

Fixes: af4b1102 ("net: sparx5: add ethtool configuration and statistics support")
Fixes: b37a1bae ("net: sparx5: add mactable support")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>