megaraid_sas: Initialize tasklet before setting up IRQs

It may happen (kdump), that an interrupt is invoked just after the
setup_irqs function was called but before the tasklet was initialised.
At this phase the hw ints should have been disabled, but for unknown
reason this mechanism seems to not work properly.

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