USB: drivers/block/ub.c: use list_for_each_entry()

Low performance USB storage driver: Use list_for_each_entry() instead
of list_for_each()

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>