Bluetooth: 6lowpan: Remove PSM setting code

Removing PSM setting debugfs interface as the IPSP has a well
defined PSM value that should be used.

The patch introduces enable flag that can be used to toggle
6lowpan on/off.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>