scsi: megaraid_sas: Create separate functions to allocate ctrl memory

No functional change. Code refactoring to improve readability.  Move the
code to allocate and free controller memory into separate functions.

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