rapidio: use kobj_to_dev()

Use kobj_to_dev() instead of open-coding it.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: "Bounine, Alexandre" <Alexandre.Bounine@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>