libata-scsi: Fixup ata_gen_passthru_sense()

There's a typo in ata_gen_passthru_sense(), where the first byte
would be overwritten incorrectly later on.

Reported-by: Charles Machalow <csm10495@gmail.com>
Signed-off-by: Hannes Reinecke <hare@suse.com>
Fixes: 11093cb1 ("libata-scsi: generate correct ATA pass-through sense")
Cc: stable@vger.kernel.org # v4.7+
Signed-off-by: Tejun Heo <tj@kernel.org>