Bluetooth: 6lowpan: Rename ambiguous variable

This patch renames the variable used to trigger scheduling of
delete_netdev. Changed to infinitiv in order to describe the action
to be done.

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>