scsi: hisi_sas: Introduce hisi_sas_phy_set_linkrate()

There is much common code and functionality between the HW versions to set
the PHY linkrate.

As such, this patch factors out the common code into a generic function
hisi_sas_phy_set_linkrate().

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>