[libata] disable clustering by default, whitespace cleanups

Disable SCSI's clustering of S/G entries, typically reducing overall
S/G list size.  Although ATA S/G tables are getting smarter (64-bit
address, 32-bit length, no DMA boundary besides 4G itself), working
harder to generate large byte sizes for each S/G size is not necessarily
in our best interest:

* SATA consists of frames, much like ethernet, called "FIS".  A
  Data FIS consists of up to 2048 dwords (8K) of data.  We don't want
  generate large byte sizes in S/G entries, just to have the host controller
  break them up again into smaller chunks.
* Reduces the possibility that ata_fill_sg (libata-core.c) must split
  an S/G into two pieces, because it straddles a 64K boundary.
* Reduces the possibility that FIS-related errata workarounds
  must be enabled.  One such exists in the Silicon Image driver (sata_sil),
  which is currently worked around by limiting the max-sectors-per-request
  to 15.

include/linux/libata.h

@@ -28,7 +28,6 @@
 #include <asm/io.h>
 #include <linux/ata.h>
 
-
 /*
  * compile-time options
  */
@@ -82,7 +81,7 @@ enum {
 	ATA_SHT_EMULATED	= 1,
 	ATA_SHT_CMD_PER_LUN	= 1,
 	ATA_SHT_THIS_ID		= -1,
-	ATA_SHT_USE_CLUSTERING	= 1,
+	ATA_SHT_USE_CLUSTERING	= 0,
 
 	/* struct ata_device stuff */
 	ATA_DFLAG_LBA48		= (1 << 0), /* device supports LBA48 */