scsi: csiostor: Fix wrong return value in csio_hw_prep_fw()

On an error exit path, a negative error code should be returned instead of
a positive return value.

Link: https://lore.kernel.org/r/20200802111531.5065-1-tianjia.zhang@linux.alibaba.com


Fixes: f40e74ff ("csiostor:firmware upgrade fix")
Cc: Praveen Madhavan <praveenm@chelsio.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>