Bluetooth: BAP: Improve handling of ASCS notification error

If the num_ase == 0xff then it is a special case that needs to be
handled like if num_ase == 0x01.

If there is an error with ase_id = 0x00 then the error cannot
be translated to a specific stream, so the callbacks may now get
NULL for the stream object.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>