scsi: gdth: Remove set but used 'cmd_index'

scsi: gdth: Remove set but used 'cmd_index'

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

drivers/scsi/gdth.c: In function ‘gdth_async_event’:
drivers/scsi/gdth.c:3010:9: warning: variable ‘cmd_index’ set but not
used [-Wunused-but-set-variable]
     int cmd_index;

Link: https://lore.kernel.org/r/20200909082627.101984-1-yebin10@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>