Bluetooth: mgmt: Implement Set LE command

This patch implements support for the Set LE mgmt command. Now, in
addition to the enable_le module parameter user space needs to send an
explicit Enable LE command to enable LE support.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>