scsi: megaraid_sas: Call disable_irq from process IRQ poll

On PowerPC architecture, calling disable_irq_nosync from IRQ context is not
providing the required effect.

In current megaraid_sas driver, disable_irq_nosync is being called from IRQ
context before enabling IRQ poll. But due to the issue seen on PPC, after
IRQ poll disable and legacy ISR is enabled, we are not seeing our ISR
getting called.

Fix: Call disable_irq from IRQ poll thread context instead of IRQ context.

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