USB: core: no need to save usb_devices_root

There is no need to save the usb_devices debugfs file as we only need it
when removing it, so have the debugfs code look it up when it is needed
instead, saving the storage.

Link: https://lore.kernel.org/r/YCubCA/trHAF7PtF@kroah.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>