[SCSI] megaraid_sas: add readl to force PCI posting flush

MegaRAID SAS Driver get unexpected Interrupt.  Add the dummy readl to
force PCI flush will fix this issue.

Signed-off-by: Bo Yang <bo.yang@lsi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>