scsi: megaraid_sas: Rework device add code in AEN path

In preparation of adding support for the DEVICE_LIST DCMD, this patch
refactors the code in the AEN event handling path.  Add new function to
update the PD and LD list in driver.  Move the code to scan PD and VD
channels into separate function.

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