scsi: megaraid_sas: Rework code to get PD and LD list

During FW init, combine the code to get the PD and LD list from FW into a
single function.  This patch is in preparation for adding support for
HOST_DEVICE_LIST DCMD.

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>