Bluetooth: Use an async request for mgmt_set_connectable

This patch changes the mgmt_set_connectable handler to use an async
request for sending the required HCI command. This is necessary
preparation for handling the fast connectable change that needs to be
associated with disabling the connectable setting.

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>