watchdog: Use kobj_to_dev() API

Use kobj_to_dev() API instead of container_of().

Signed-off-by: Wang Qing <wangqing@vivo.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1591945384-14587-1-git-send-email-wangqing@vivo.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>