power: supply: cw2015: Fix potential null-ptr-deref in cw_bat_probe()

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

power: supply: cw2015: Fix potential null-ptr-deref in cw_bat_probe()

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

cw_bat_probe()
create_singlethread_workqueue() # failed, cw_bat->wq 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.