USB: dwc3: remove debugfs root dentry storage

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 <jackp@codeaurora.org>
Reviewed-by: Peter Chen <peter.chen@kernel.org>
Acked-by: Felipe Balbi <balbi@kernel.org>
Link: https://lore.kernel.org/r/20210609093924.3293230-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>