Bluetooth: audio: ascs: Reject peer Release operation in Idle state

This fixes crash that happened when client initiated Release operation
on ASE in Idle state. In such case the operation shall be rejected with
Invalid ASE State Machine Response_Code.

Fixes: ASCS/SR/SPE/BI-10-C
Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>