Bluetooth: Add per-instance adv disable/remove

Add functionality to disable and remove advertising instances,
and use that functionality in MGMT add/remove advertising calls.

Currently, advertising is globally-disabled, i.e. all instances are
disabled together, even if hardware offloading is available. This
patch adds functionality to disable and remove individual adv
instances, solving two issues:

1. On new advertisement registration, a global disable was done, and
then only the new instance was enabled. This meant only the newest
instance was actually enabled.

2. On advertisement removal, the structure was removed, but the instance
was never disabled or removed, which is incorrect with hardware offload
support.

Signed-off-by: Daniel Winkler <danielwinkler@google.com>
Reviewed-by: Shyh-In Hwang <josephsih@chromium.org>
Reviewed-by: Alain Michaud <alainm@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>