Bluetooth: Fix fast connectable response sending

The mgmt_set_fast_connectable response should be sent only when all
related HCI commands have completed. This patch fixes the issue by using
an async request and sending the response to user space throught the
complete callback of the request. The patch also fixes in the same go
the return parameters of the command which should be the current
settings.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>