w1: fix deadloop in __w1_remove_master_device()

I got a deadloop report while doing device(ds2482) add/remove test:

  [  162.241881] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  [  163.272251] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  [  164.296157] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  ...

__w1_remove_master_device() can't return, because the dev->refcnt is not zero.

w1_add_master_device()			|
  w1_alloc_dev()			|
    atomic_set(&dev->refcnt, 2)		|
  kthread_run()				|
					|__w1_remove_master_device()
					|  kthread_stop()
  // KTHREAD_SHOULD_STOP is set,	|
  // threadfn(w1_process) won't be	|
  // called.				|
  kthread()				|
					|  // refcnt will never be 0, it's deadloop.
					|  while (atomic_read(&dev->refcnt)) {...}

After calling w1_add_master_device(), w1_process() is not really
invoked, before w1_process() starting, if kthread_stop() is called
in __w1_...