lms2012: exit-script: use poweroff to power off the EV3

Now that poweroff does work, use it.
d_power: Disable power off when module is unloaded

Remove that violent piece of code which does look like if someone
hits a mechanical button over and over in order to make it work.
We live in the digital age.
Merge pull request #23 from aholler/master

power off finalized