Bluetooth: MGMT start discovery LE-Only support

This patch adds LE-Only discovery procedure support to MGMT Start
Discovery command.

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