scsi: megaraid-sas: request irqs later

It is not good when an irq arrives before driver structures are
allocated.

Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>