usb: wusbcore: resource cleanup fix in __wa_xfer_setup_segs

This patch updates __wa_xfer_setup_segs error path to only clean up the
xfer->seg entry that it failed to create and then set that entry to
NULL.  wa_xfer_destroy will clean up the remaining xfer->segs that were
fully created.  It also moves the code to create the dto sg list to an
out of line function to make __wa_xfer_setup_segs easier to read.  Prior
to this change, __wa_xfer_setup_segs would clean up all entries in the
xfer->seg array in case of an error but it did not set them to NULL.
This resulted in a double free when wa_xfer_destroy was eventually
called by the higher level error handler.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>