scsi: ibmvscsis: Fix write_pending failure path

For write_pending if the queue is down or client failed then return -EIO
so that LIO can properly process the completed command. Prior we
returned 0 since LIO could not handle it properly. Now with commit
fa7e25cf

 ("target: Fix unknown fabric callback queue-full errors")
that patch addresses LIO's ability to handle things right.

Signed-off-by: Bryant G. Ly <bgly@us.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>