scsi: core: use kobj_to_dev

Use kobj_to_dev to instead of open-coding it.

Link: https://lore.kernel.org/r/20200225100411.10250-1-guosongsu@gmail.com


Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Guosong Su <suguosong@xiaomi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>