LE Audio: Fix response code expected by ASCS/SR/SPE/BI-18-C

When handling ASE Control operations requested by client,
first verify that the requested ID is within the allowed range.
If it does, lets assume that the NULL returned from ase_find()
means the ASE state is set to idle.

This will fix ASE Notification Response code expected by
the ASCS/SR/SPE/BI-18-C PTS test case.

Signed-off-by: Magdalena Kasenberg <magdalena.kasenberg@codecoup.pl>