scsi: megaraid_sas: Export RAID map id through sysfs

Add a sysfs interface to get the raid map index that is being used by
driver.

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