scsi: fnic: Fix for "Number of Active IOs" in fnicstats becoming negative

Fixing the IO stats update (Active IOs and IO completion) to prevent
"Number of Active IOs" from becoming negative in the fnistats output.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Signed-off-by: Sesidhar Baddela <sebaddel@cisco.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>