Bluetooth: Move HCI_ADVERTISING handling into mgmt.c

We'll soon need to make decisions on toggling the HCI_ADVERTISING flag
based on pending mgmt_set_powered commands. Therefore, move the handling
from hci_event.c into mgmt.c.

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