ihdad: graceful failure on DSP/cAVS platforms (Phase 8.1)

On cAVS/SOF platforms (e.g. LG Gram 8086:7728), STATESTS reads 0
because the codec sits behind the audio DSP; the legacy HDA codec-wake
mechanism cannot reach it. enumerate() previously hardcoded codec 0 and
would hang reading a non-existent codec via CORB/RIRB. Now, if no
codecs respond after reset_controller(), log the DSP-platform diagnosis
and fail honestly with ENODEV instead of hanging. A SOF/cAVS driver
(Phase 8.2) is required for these platforms.

drivers/audio/ihdad/src/hda/device.rs

@@ -213,6 +213,18 @@ impl IntelHDA {
         module.init()?;
 
         module.info();
+
+        if module.codecs.is_empty() {
+            // DSP/cAVS platform: STATESTS=0 (codec behind DSP), enumerate()
+            // would hang on non-existent codec 0. Fail honestly.
+            log::error!(
+                "ihdad: no codecs responded on STATESTS — this is a DSP/cAVS \
+                 platform (codec behind the DSP). Legacy HDA cannot drive it; \
+                 a SOF/cAVS driver is required (Phase 8.2)."
+            );
+            return Err(Error::new(ENODEV));
+        }
+
         module.enumerate()?;
 
         module.configure()?;