Bluetooth: Refactor fast connectable HCI commands

This patch refactors the fast connectable HCI commands into their own
HCI function. This is necessary so that the same function can be reused
fo the fast connectable change required by 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>