Bluetooth: Clean up current advertising instance tracking

We can simplify a lot of code by making sure hdev->cur_adv_instance is
always up-to-date. This allows e.g. the removal of the
get_current_adv_instance() helper function and the special
HCI_ADV_CURRENT value. This patch also makes selecting instance 0x00
explicit in the various calls where advertising instances aren't
enabled, e.g. when HCI_ADVERTISING is set or we've just finished
enabling LE.

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