Input: ads7846 - use kobj_to_dev() API

Use kobj_to_dev() instead of container_of().

Signed-off-by: Wang Qing <wangqing@vivo.com>
Link: https://lore.kernel.org/r/1604893436-20206-1-git-send-email-wangqing@vivo.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>