firmware: cirrus: cs_dsp: Initialize debugfs_root to invalid

Initialize debugfs_root to -ENODEV so that if the client never sets a
valid debugfs root the debugfs files will not be created.

A NULL pointer passed to any of the debugfs_create_*() functions means
"create in the root of debugfs". It doesn't mean "ignore".
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://msgid.link/r/20240307105353.40067-1-rf@opensource.cirrus.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/firmware/cirrus/cs_dsp.c

@@ -522,7 +522,7 @@ void cs_dsp_cleanup_debugfs(struct cs_dsp *dsp)
 {
 	cs_dsp_debugfs_clear(dsp);
 	debugfs_remove_recursive(dsp->debugfs_root);
-	dsp->debugfs_root = NULL;
+	dsp->debugfs_root = ERR_PTR(-ENODEV);
 }
 EXPORT_SYMBOL_NS_GPL(cs_dsp_cleanup_debugfs, FW_CS_DSP);
 #else
@@ -2246,6 +2246,11 @@ static int cs_dsp_common_init(struct cs_dsp *dsp)
 
 	mutex_init(&dsp->pwr_lock);
 
+#ifdef CONFIG_DEBUG_FS
+	/* Ensure this is invalid if client never provides a debugfs root */
+	dsp->debugfs_root = ERR_PTR(-ENODEV);
+#endif
+
 	return 0;
 }