sg: fix read() error reporting

Fix SCSI generic read() incorrectly returning success after detecting an
error.

Cc: <stable@vger.kernel.org>
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>