soundwire: debugfs: initialize firmware_file to empty string

Passing NULL to debugfs_create_str() causes a NULL pointer dereference,
and creating debugfs nodes with NULL string pointers is no longer
permitted.

Additionally, firmware_file is a global pointer. Previously, adding every
new slave blindly overwrote it with NULL.

Fix these issues by initializing firmware_file to an allocated empty
string once in the subsystem init path (sdw_debugfs_init), and freeing
it in the exit path. Existing driver code handles empty strings
correctly.

Fixes: fe46d2a4301d ("soundwire: debugfs: add interface to read/write commands")
Reported-by: yangshiguang <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Gui-Dong Han <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/soundwire/debugfs.c

···
 	debugfs_create_file("go", 0200, d, slave, &cmd_go_fops);
 
 	debugfs_create_file("read_buffer", 0400, d, slave, &read_buffer_fops);
-	firmware_file = NULL;
-	debugfs_create_str("firmware_file", 0200, d, &firmware_file);
+	if (firmware_file)
+		debugfs_create_str("firmware_file", 0200, d, &firmware_file);
 
 	slave->debugfs = d;
 }
···
 
 void sdw_debugfs_init(void)
 {
+	if (!firmware_file)
+		firmware_file = kstrdup("", GFP_KERNEL);
+
 	sdw_debugfs_root = debugfs_create_dir("soundwire", NULL);
 }
 
 void sdw_debugfs_exit(void)
 {
 	debugfs_remove_recursive(sdw_debugfs_root);
+	kfree(firmware_file);
+	firmware_file = NULL;
 }