scsi: isci: Remove set but not used 'index'

scsi: isci: Remove set but not used 'index'

This addresses the following gcc warning with "make W=1":

drivers/scsi/isci/host.c: In function ‘sci_controller_complete_io’:
drivers/scsi/isci/host.c:2674:6: warning: variable ‘index’ set but not
used [-Wunused-but-set-variable]
 2674 |  u16 index;
      |      ^~~~~

Link: https://lore.kernel.org/r/20200827125851.428071-1-yanaijie@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>