hwmon: (adt7470) Allow faster removal

adt7470_remove will wait for the update thread to complete before
returning. This had a worst-case time of up to the user-configurable
auto_update_interval.

Replace msleep_interruptible with set_current_state and schedule_timeout
so that kthread_stop will interrupt the sleep.

Signed-off-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>