sd-device: generate e better error code when trying to allocate sd_device for non-dir

sd-device: generate e better error code when trying to allocate sd_device for non-dir

Currently, for sysfs paths outside of /sys/devices/ we do better
checking if something is a suitable path: we check if it's actually a
directory, and if not return ENODEV.

Let's make the codepath for nodes *inside* of /sys/device/ similar:
let's also return ENODEV if the path supplied is not a directory.

Previously, we'd return ENOTDIR in that case, which is quite confusing I
think.