scsi_transport_sas: always pass 0 error to blk_end_request_all

The SAS transport queues are only used by bsg, and bsg always looks at
the scsi_request results and never add the error passed in the end_io
callback.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>