Bluetooth: 6lowpan: Fix module refcount

This patch removes the additional module_put() in disconnect_all_peers()
making a correct module refcount so that the module can be removed after
disabling 6lowpan through debugfs.

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>