megaraid: fix irq setup process regression

This fixes a regression caused by commit
d3557fc8 megaraid_sas : Add separate function
for setting up IRQs This makes boot end with 'root does not exist' message on
certain adapters.

The bug is that the driver does not setup ints for cards without msi-x
support.  This patch fixes it, in addition to that it moves tasklet
initialisation before enable_intr, otherwise a kernel panic may occur, when an
interrupt arrives before the tasklet is ready.

Signed-off-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>