Bluetooth: Don't clear HCI_ADVERTISING when powering off

Once mgmt_set_powered(off) is updated to clear the scan mode we should
not just blindly clear the HCI_ADVERTISING flag in mgmt_advertising()
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>