Bluetooth: Add proper flag for fast connectable mode

In order to be able to represent fast connectable mode in the mgmt
settings we need to have a HCI dev flag for it. This patch adds the flag
and makes sure its value is changed whenever a mgmt_set_fast_connectable
command completes.

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>