Bluetooth: audio: ascs: Refactor ASE Release handler

This removes code duplicates.
The ASE Release operation handler has been refactored by moving the state
validation check to the ase_release function body and providing the
`rsp` pointer that is later filled. ascs_cp_rsp_add is not called from
ase_release explicitly, so the contents of the respons buffer are not
changed in case the ase_release is called from non control point context
e.g. `disconnected` callback.

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