ieee802154: 6lowpan: fix packet layer registration

This patch fixes 802.15.4 packet layer registration when mutliple
lowpan interfaces will be added. We need to register the packet layer at
the first lowpan interface and deregister it at the last interface. This
done by open_count variable which is protected by rtnl.

Additional do a quiet fix by adding dev_put(real_dev) when netdev
registration fails, which fix the refcount for the wpan dev.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>