Bluetooth: Move more logic into set_connectable complete callback

This patch moves the responsibility of setting/clearing the
HCI_CONNECTABLE flag to the request completion callback of the Set
Connectable command. This will allow us to cleanly add support for LE
Advertising hooks in later patches.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>