The clear_adv_instance() function could not clean up multiple
advertising instances previously. It is being changed to provide both, a
means to clean up a single instance and cleaning up all instances at
once.

An additional instance parameter is being introduced to achieve this.
Passing in 0x00 to this parameter signifies that all instances should be
cleaned up. This semantics has been chosen similarly to the semantics of
the instance parameter in the remove_advertising() function.

When removing a single instance the method also ensures that another
instance will be scheduled if available. When the currently advertising
method is being removed, it will be canceled immediately.

Signed-off-by: Florian Grandel <fgrandel@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>