Bluetooth: Audio: Fix MCC disconnect issue

When MCC disconnected, it would memset the entire instance.
This causes issues, because the subscription parameters
would also be memset, causing the `notify` callback to
be set to NULL. If this was done as part of a disconnect,
the GATT layer would attempt to call the notify callback
as part of cleaning up the ACL, but if the callback was
set to NULL (from the memset) this caused a fatal error.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>