USB: fix bug in sg initialization in usbtest

This patch (as1062) fixes a bug in the scatter-gather initialization
code in the usbtest driver.  When the sg-helper conversion was
performed, it wasn't done correctly.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>