[SCSI] mpt2sas : the sanity check in base_interrupt needs to be on dword boundary

The poison sanity check on the reply_post_free register needs to be by 32bit,
not 64bit. The poison check is there because its possible that the driver read
the 1st 32bit before the 2nd 32bit has been written to by firmware.  In other
words, this handles race between driver reading the 64 bit register, and it
being dma'd across pci memory from controller firmware as two 32bit pci writes.

Signed-off-by: Eric Moore <eric.moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>