USB: use standard SG iterator in the scatter-gather library

This patch (as1103) changes the iteration in the USB scatter-gather to
use a standard SG iterator.  Otherwise the iteration will fail if it
encounters a chained SG list.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>