[libata] linux/libata.h: reorganize ata_device struct members a bit

Put the big stuff at the end, to prepare for upcoming changes (and
also hopefully achieve nicer packing of remaining members).
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

include/linux/libata.h

@@ -548,11 +548,6 @@ struct ata_device {
 	u64			n_sectors;	/* size of device, if ATA */
 	unsigned int		class;		/* ATA_DEV_xxx */
 
-	union {
-		u16		id[ATA_ID_WORDS]; /* IDENTIFY xxx DEVICE data */
-		u32		gscr[SATA_PMP_GSCR_DWORDS]; /* PMP GSCR block */
-	};
-
 	u8			pio_mode;
 	u8			dma_mode;
 	u8			xfer_mode;
@@ -574,8 +569,13 @@ struct ata_device {
 	u16			sectors;	/* Number of sectors per track */
 
 	/* error history */
-	struct ata_ering	ering;
 	int			spdn_cnt;
+	struct ata_ering	ering;
+
+	union {
+		u16		id[ATA_ID_WORDS]; /* IDENTIFY xxx DEVICE data */
+		u32		gscr[SATA_PMP_GSCR_DWORDS]; /* PMP GSCR block */
+	};
 };
 
 /* Offset into struct ata_device.  Fields above it are maintained