staging: fieldbus: use kobj_to_dev() to get device

Use kobj_to_dev() instead of container_of()

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20201117025933.668938-1-dingxiang@cmss.chinamobile.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>