scsi: mpt3sas: Added print to notify cable running at a degraded speed.

Driver processes the event MPI26_EVENT_ACTIVE_CABLE_DEGRADED when a
cable is present and is running at a degraded speed (below the SAS3 12
Gb/s rate). Prints added to inform the user that the cable is not
running at optimal speed.

Signed-off-by: Chaitra P B <chaitra.basappa@broadcom.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>