Bluetooth: ascs: Avoid possible unexpected assert

This avoids unexpected assert that may happen when the client tries to
QoS configure ASE that is in state which does not allow to be configured.
In such case the assert shall not be not be triggered, as it's not stack
fauly. The assert check has been moved after the state check, so the ASCS
implementation will just return an error code to the client.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>