Bluetooth: Convert Set SC to use HCI Request

This patch converts the Set Secure Connection HCI handling to use a HCI
request instead of using a hard-coded callback in hci_event.c. This e.g.
ensures that we don't clear the flags incorrectly if something goes
wrong with the power up process (not related to a mgmt Set SC command).

The code can also be simplified a bit since only one pending Set SC
command is allowed, i.e. mgmt_pending_foreach usage is not needed.

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