This change is about removing three fields from "struct usb2_xfer"
which can be reached from "struct usb2_xfer_root" instead and cleaning
up the code after this change. The fields are "xfer->udev",
"xfer->xfer_mtx" and "xfer->usb2_sc". In this process the following
changes were also made:
Rename "usb2_root" to "xroot" which is short for "xfer root".
Rename "priv_mtx" to "xfer_mtx" in USB core.
The USB_XFER_LOCK and USB_XFER_UNLOCK macros should only be used in
the USB core due to dependency towards "xroot". Substitute macros
for the real lock in two USB device drivers.
Submitted by: Hans Petter Selasky
13bd84042e