6lowpan: fix debugfs interface entry name

This patches moves the debugfs interface related register after
netdevice register. The function lowpan_dev_debugfs_init will use
"dev->name" which can be before register_netdevice a format string.
The function register_netdevice will evaluate the format string if
necessary and replace "dev->name" to the real interface name.

Reported-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>