libata: use __scsi_format_command()

libata already uses an internal buffer, so we should be using
__scsi_format_command() here.

Tested-by: Robert Elliott <elliott@hp.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>