hotplug: add a debug tunable

The pre-defined value didn't satisfy my needs.
Adding a simple debug tunable to be swichtable
directly from userspace.
Warning: This might spam your kernel log ;)

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
hotplug: rename it and set proper config

Just for the looks, the internal name is still the same
to be compatible with the sysfs entries.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>