Bluetooth: LE support for MGMT stop discovery

This patch adds LE support to MGMT stop discovery command. So,
now we are able to cancel LE discovery procedures (LE-only and
interleaved).

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>