scsi: mpi3mr: Remove unnecessary cast

coccinelle reports a warning:

  WARNING: casting value returned by memory allocation function to (struct
  mpi3mr_throttle_group_info *) is useless

To fix this, the unnecessary cast is removed.

Link: https://lore.kernel.org/r/Yx+kp8NxHvDHs7dv@playground


Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>