[ata] remove 'packed' attributed from struct ata_prd

It's not needed, and it generates very poor code on some platforms.

Noticed by Bart and David Miller.

include/linux/ata.h

@@ -192,7 +192,7 @@ enum ata_ioctls {
 struct ata_prd {
 	u32			addr;
 	u32			flags_len;
-} __attribute__((packed));
+};
 
 struct ata_taskfile {
 	unsigned long		flags;		/* ATA_TFLAG_xxx */