scsi: tcmu: remove useless code and clean up the code style.

Since the TCMU_RING_SIZE macro is not using here will discard it and at the
same time clean up the code style.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>