usb: wusbcore: rename fields in struct wahc

Rename xfer_result to dti_buf and xfer_result_size to dti_buf_size in
struct wahc.  The dti buffer will also be used for isochronous status
packets once isochronous transfers are supported.

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