scsi: mpi3mr: Rework mrioc->bsg_device model to fix warnings

scsi: mpi3mr: Rework mrioc->bsg_device model to fix warnings

During driver unload, mrioc->bsg_device reference count becomes
negative. Also, as reported in [1], the driver's bsg_device model had few
more bugs. Fix all these up.

[1] https://marc.info/?l=linux-scsi&m=165183971411991&w=2

Link: https://lore.kernel.org/r/20220526170157.58274-1-sumit.saxena@broadcom.com
Fixes: 233bdd00449c ("scsi: mpi3mr: Add bsg device support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>