Driver core: allow devices in classes to have no parent

This fixes an oops when a device is attached to a class, yet has no
"parent" device.  An example of this would be the "lo" device in the
network core.

We should create a "virtual" subdirectory under /sys/devices/ for these,
but no one seems to agree on a proper name for it yet...

Oh, and update my copyright on the driver core.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>