scsi: pm80xx: Do not request 12G sas speeds

Occasionally, 6G capable drives fail to train at 6G on links that look good
from a signal-integrity perspective. PMC suggests configuring the port to
not even expect 12G.

Link: https://lore.kernel.org/r/20191114100910.6153-11-deepak.ukey@microchip.com


Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: peter chang <dpf@google.com>
Signed-off-by: Deepak Ukey <deepak.ukey@microchip.com>
Signed-off-by: Viswas G <Viswas.G@microchip.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>