Bluetooth: Don't clear HCI_DISCOVERABLE when powering off

Once mgmt_set_powered(off) is updated to clear the scan mode we should
not just blindly clear the HCI_DISCOVERABLE flag in mgmt_discoverable()
but first check if there is a pending set_powered operation.

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