Bluetooth: Update stop_discovery to use HCI request

This patch modifies the stop_discovery function so it uses the HCI
request framework.

The HCI request is built according to the current discovery state
(inquiry, LE scanning or name resolving) and a complete callback is
register to handle the command complete event for the stop discovery
command. This way, we move all stop_discovery mgmt handling code
spread in hci_event.c to a single place in mgmt.c.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>