USB: dwc3: fix memory leak with using debugfs_lookup()

In the Linux kernel, the following vulnerability has been resolved:

USB: dwc3: fix memory leak with using debugfs_lookup()

When calling debugfs_lookup() the result must have dput() called on it, otherwise the memory will leak over time. To make things simpler, just call debugfs_lookup_and_remove() instead which handles all of the logic at once.

Note, the root dentry for the debugfs directory for the device needs to be saved so we don't have to keep looking it up, which required a bit more refactoring to properly create and remove it when needed.