xhci: fix debugfs register accesses while suspended

Wire up the debugfs regset device pointer so that the controller is
resumed before accessing registers to avoid crashing or locking up if it
happens to be runtime suspended.

Fixes: 02b6fdc2 ("usb: xhci: Add debugfs interface for xHCI driver")
Cc: stable@vger.kernel.org # 4.15: 30332eee

: debugfs: regset32: Add Runtime PM support
Cc: stable@vger.kernel.org # 4.15
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20230405090342.7363-1-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>