ata: libata-pmp: disable the SEMB port on Marvell 88SM9705

The Marvell 88SM9705 is a 1-to-5 SATA port multiplier with one
additional SEMB (SATA Enclosure Management Bridge) port [1].  Ports
0-4 are device ports and port 5 is the SEMB.  The number of fan-out
ports in GSCR[2] is 6 when the SEMB is enabled and 5 when it is not,
so on boards without the SEMB there is no link 5 and this quirk does
nothing.  That includes the common 8-port PCIe cards that pair an
88SE9215 with an 88SM9705, which report 5 ports.

The SEMB port never answers SRST.  libata probes it like a device
port, and every EH round on the PMP ends in a retry loop that lasts
about a minute before EH gives up, then starts over:

  ata19.15: Port Multiplier 1.2, 0x1b4b:0x9705 r160, 6 ports, feat 0x1/0x1f
  ata19.05: hard resetting link
  ata19.05: failed to resume link (SControl 0)
  ata19.05: softreset failed (1st FIS failed)
  ata19.05: hard resetting link
  ...
  ata20.05: reset failed, giving up

Disk discovery waits for this, including disks on other controllers.

libata.force only partly works around it.  The "disable" horkage
only takes effect after IDENTIFY, which the SEMB link never reaches.
"norst" on port 5 stops the retry loop, but the disks still come up
about a minute later than with this change (see below).

Treat port 5 the same way as port 4 of the Marvell 88SM4140 and mark
the link ATA_LFLAG_DISABLED.

Tested on a Synology DX1222 expansion unit (four 88SM9705 r160 behind
a Marvell 88SE9235, passed through to a KVM guest, 6 disks), 7.4-rc1
with and without this change, one boot each:

                                    softreset  reset failed,  last disk
                                    failed     giving up      attached
  7.4-rc1, no libata.force          60         12             197 s
  7.4-rc1, libata.force=N.05:norst   0          0              70 s
  7.4-rc1 + this patch, no force     0          0              11 s

The problem was first found on a Synology DX1215 with the same chain,
where this change brought the disks up after about 20 s instead of
about 200 s, including a disk on a separate AHCI controller.

Each PMP still logs one "failed to resume link (SControl 0)" for port 5
at probe time.

Link: https://www.marvell.com/content/dam/marvell/en/public-collateral/storage/marvell-storage-88sm9705-functional-specifications-2015-06.pdf [1]
Cc: stable@vger.kernel.org
Tested-by: Kyrylo Biryuk <illofspeed@gmail.com>
Signed-off-by: Marco Ziliani <rylos78@gmail.com>
Link: https://lore.kernel.org/r/20261107-pmp-88sm9705-v2-1-18eed983d3db@gmail.com
Signed-off-by: Niklas Cassel <cassel@kernel.org>

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