hwmon: (adt7470) No need for additional synchronization on kthread_stop()

The kthread_stop() waits for the thread to exit. There is no need for an
additional synchronization needed to join on the kthread.

The completion was added by 89fac11c ("adt7470: make automatic fan
control really work").

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Darrick J. Wong <djwong@us.ibm.com>
Cc: Jean Delvare <jdelvare@suse.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>