USB: dwc3: remove debugfs root dentry storage

There is no need to keep around the debugfs "root" directory for the
dwc3 device. Instead, look it up anytime we need to find it. This will
help when callers get out-of-order and we had the potential to have a
"stale" pointer around for the root dentry, as has happened in the past.

Tested-by: Jack Pham <[e-mail]>
Reviewed-by: Peter Chen <[e-mail]>
Acked-by: Felipe Balbi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/dwc3/core.h

···
  * @link_state: link state
  * @speed: device speed (super, high, full, low)
  * @hwparams: copy of hwparams registers
- * @root: debugfs root folder pointer
  * @regset: debugfs pointer to regdump file
  * @dbg_lsp_select: current debug lsp mux register selection
  * @test_mode: true when we're entering a USB test mode
···
 	u8			num_eps;
 
 	struct dwc3_hwparams	hwparams;
-	struct dentry		*root;
 	struct debugfs_regset32	*regset;
 
 	u32			dbg_lsp_select;

drivers/usb/dwc3/debugfs.c

···
 void dwc3_debugfs_create_endpoint_dir(struct dwc3_ep *dep)
 {
 	struct dentry		*dir;
+	struct dentry		*root;
 
-	dir = debugfs_create_dir(dep->name, dep->dwc->root);
+	root = debugfs_lookup(dev_name(dep->dwc->dev), usb_debug_root);
+	dir = debugfs_create_dir(dep->name, root);
 	dwc3_debugfs_create_endpoint_files(dep, dir);
 }
 
···
 	dwc->regset->base = dwc->regs - DWC3_GLOBALS_REGS_START;
 
 	root = debugfs_create_dir(dev_name(dwc->dev), usb_debug_root);
-	dwc->root = root;
-
 	debugfs_create_regset32("regdump", 0444, root, dwc->regset);
 	debugfs_create_file("lsp_dump", 0644, root, dwc, &dwc3_lsp_fops);
 
···
 
 void dwc3_debugfs_exit(struct dwc3 *dwc)
 {
-	debugfs_remove_recursive(dwc->root);
+	debugfs_remove(debugfs_lookup(dev_name(dwc->dev), usb_debug_root));
 	kfree(dwc->regset);
 }

drivers/usb/dwc3/gadget.c

···
 			list_del(&dep->endpoint.ep_list);
 		}
 
-		debugfs_remove_recursive(debugfs_lookup(dep->name, dwc->root));
+		debugfs_remove_recursive(debugfs_lookup(dep->name,
+				debugfs_lookup(dev_name(dep->dwc->dev),
+					       usb_debug_root)));
 		kfree(dep);
 	}
 }