sg: fix read() error reporting

commit 3b524a683af8991b4eab4182b947c65f0ce1421b upstream.

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

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>