xhci: Fix xhci debugfs NULL pointer dereference in resume from hibernate

Free the virt_device and its debugfs_private member together.

When resuming from hibernate the .free_dev callback unconditionally
freed the debugfs_private member, but could leave virt_device intact.

This triggered a NULL pointer dereference after resume when usbmuxd
sent a USBDEVFS_SETCONFIGURATION ioctl to a device, trying to add a
endpoint debugfs entry to a already freed debugfs_private pointer.

Fixes: 02b6fdc2

 ("usb: xhci: Add debugfs interface for xHCI driver")
Reported-by: Alexander Kappner <agk@godking.net>
Tested-by: Alexander Kappner <agk@godking.net>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>