Bluetooth: Clean up advertising initialization in powered_update_hci()

The logic in powered_update_hci() to initialize the advertising data &
state is a bit more complicated than it needs to be. It was previously
not doing anything if HCI_LE_ENABLED wasn't set, but this was not
obvious by quickly looking at the code. Now the conditions for the
various actions are more explicit. Another simplification is due to
the fact that __hci_req_schedule_adv_instance() takes care of setting
hdev->cur_adv_instance so there's no need to set it before calling the
function.

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