[SCSI] mpt3sas: cut and paste bug storing trigger mpi

ioc->diag_trigger_mpi is an SL_WH_MPI_TRIGGERS_T struct.

There is a cut and paste error here and SL_WH_EVENT_TRIGGERS_T is used
instead of SL_WH_MPI_TRIGGERS_T.  Since the SL_WH_EVENT_TRIGGERS_T is
smaller than SL_WH_MPI_TRIGGERS_T, it means we only clear part of the
buffer.

I've changed it to use sizeof(ioc->diag_trigger_mpi) which is a bit
simpler.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: "Sreekanth Reddy" <Sreekanth.reddy@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>