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.

Change-Id: I7fd1973704cd2a9e193fd77e01f7e09236f41117
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>
Git-commit: 8562d5bfc0fcdfd3aef32991e17dca585ae5ae7d
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[quic_ppratap@quicinc.com: resolved trivial merge conflicts]
Signed-off-by: Pratham Pratap <quic_ppratap@quicinc.com>