Bluetooth: Use HCI request in interleaved discovery

In order to have a better HCI error handling in interleaved discovery
functionality, we should use the HCI request framework.

This patch updates le_scan_disable_work function so it uses the
HCI request framework instead of the hci_send_cmd helper. A complete
callback is registered (le_scan_disable_work_complete function) so we
are able to trigger the inquiry procedure (if we are running the
interleaved discovery) or to stop the discovery procedure (if we are
running LE-only discovery).

This patch also removes the extra logic in hci_cc_le_set_scan_enable
to trigger the inquiry procedure and the mgmt_interleaved_discovery
function since they become useless.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>