scsi: mpi3mr: Fix warnings reported by smatch

scsi: mpi3mr: Fix warnings reported by smatch

Fix the following warning reported by static analysis tool smatch:

smatch warnings:
drivers/scsi/mpi3mr/mpi3mr_os.c:873 mpi3mr_update_tgtdev() error: we
previously assumed 'mrioc->shost' could be null (see line 870

Link: https://lore.kernel.org/r/20210629141153.3158-1-sreekanth.reddy@broadcom.com
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>