Bluetooth: 6lowpan: Move netdev sysfs device reference

This patch moves the sysfs device used by the netdev from the device of
the first connected peer to the hci sysfs device. Using the sysfs device
of hci instead of the first connected device fixes this issue such that
the sysfs group of tx-0 and bt0 kobject are still present after the last
peer has been deleted and all sysfs entries can be removed.

Signed-off-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: Glenn Ruben Bakke <glenn.ruben.bakke@nordicsemi.no>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>