ahci: disabled FBS prior to issuing software reset

Tested with Marvell 88se9125, attached with one port mulitplier(5 ports)
and one disk, we will get following boot log messages if using current
code:

  ata8: SATA link up 6.0 Gbps (SStatus 133 SControl 330)
  ata8.15: Port Multiplier 1.2, 0x1b4b:0x9715 r160, 5 ports, feat 0x1/0x1f
  ahci 0000:03:00.0: FBS is enabled
  ata8.00: hard resetting link
  ata8.00: SATA link down (SStatus 0 SControl 330)
  ata8.01: hard resetting link
  ata8.01: SATA link down (SStatus 0 SControl 330)
  ata8.02: hard resetting link
  ata8.02: SATA link down (SStatus 0 SControl 330)
  ata8.03: hard resetting link
  ata8.03: SATA link up 6.0 Gbps (SStatus 133 SControl 133)
  ata8.04: hard resetting link
  ata8.04: failed to resume link (SControl 133)
  ata8.04: failed to read SCR 0 (Emask=0x40)
  ata8.04: failed to read SCR 0 (Emask=0x40)
  ata8.04: failed to read SCR 1 (Emask=0x40)
  ata8.04: failed to read SCR 0 (Emask=0x40)
  ata8.03: native sectors (2) is smaller than sectors (976773168)
  ata8.03: ATA-8: ST3500413AS, JC4B, max UDMA/133
  ata8.03: 976773168 sectors, multi 0: LBA48 NCQ (depth 31/32)
  ata8.03: configured for UDMA/133
  ata8.04: failed to IDENTIFY (I/O error, err_mask=0x100)
  ata8.15: hard resetting link
  ata8.15: SATA link up 6.0 Gbps (SStatus 133 SControl 330)
  ata8.15: Port Multiplier vendor mismatch '0x1b4b' != '0x133'
  ata8.15: PMP revalidation failed (errno=-19)
  ata8.15: hard resetting link
  ata8.15: SATA link up 6.0 Gbps (SStatus 133 SControl 330)
  ata8.15: Port Multiplier vendor mismatch '0x1b4b' != '0x133'
  ata8.15: PMP revalidation failed (errno=-19)
  ata8.15: limiting SATA link speed to 3.0 Gbps
  ata8.15: hard resetting link
  ata8.15: SATA link up 3.0 Gbps (SStatus 123 SControl 320)
  ata8.15: Port Multiplier vendor mismatch '0x1b4b' != '0x133'
  ata8.15: PMP revalidation failed (errno=-19)
  ata8.15: failed to recover PMP after 5 tries, giving up
  ata8.15: Port Multiplier detaching
  ata8.03: disabled
  ata8.00: disabled
  ata8: EH complete

The reason is that current detection code doesn't follow AHCI spec:

First,the port multiplier detection process look like this:

	ahci_hardreset(link, class, deadline)
	if (class == ATA_DEV_PMP) {
		sata_pmp_attach(dev)	/* will enable FBS */
		sata_pmp_init_links(ap, nr_ports);
		ata_for_each_link(link, ap, EDGE) {
			sata_std_hardreset(link, class, deadline);
			if (link_is_online)	/* do soft reset */
				ahci_softreset(link, class, deadline);
		}
	}
But, according to chapter 9.3.9 in AHCI spec: Prior to issuing software
reset, software shall clear PxCMD.ST to '0' and then clear PxFBS.EN to
'0'.

The patch test ok with kernel 3.11.1.

tj: Patch white space contaminated, applied manually with trivial
    updates.

Signed-off-by: Xiangliang Yu <yuxiangl@marvell.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org