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

In the Linux kernel, the following vulnerability has been resolved:

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.