BugLink: http://bugs.launchpad.net/bugs/1546572

Use kobj_to_dev() instead of open-coding it.

Link: http://patchwork.freedesktop.org/patch/msgid/3fea991541fbfc4ffece2c174adeb02cb9436c90.1452696179.git.geliangtang@163.comSigned-off-by: Geliang Tang <geliangtang@163.com>
(cherry picked from commit d122cbf1)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>