Bluetooth: audio: ascs: Refactor ASE Disable handler

This removes code duplicates.
ASE Disable operation handler has been refactored by providing the
`rsp` pointer as one of the function parameters. ascs_cp_rsp_add is
not called from ase_disable explicitly, so the contents of the
response buffer are not changed in case the ase_disable is called
from non control point context.

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