net: lan966x: Fix potential null-ptr-deref in lan966x_stats_init()

lan966x_stats_init() calls create_singlethread_workqueue() and not
checked the ret value, which may return NULL. And a null-ptr-deref may
happen:

lan966x_stats_init()
    create_singlethread_workqueue() # failed, lan966x->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.

Fixes: 12c2d0a5

 ("net: lan966x: add ethtool configuration and statistics")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>