watchdog/hpwdt: Stop hpwdt on unregister.

Have the WD core stop the watchdog on unregister instead of explicitly
calling hpwdt_stop() in hpwdt_exit().

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>