[SCSI] scsi_transport_iscsi: Fix documentation os parameter

Fixes documentation of a parameter of iscsi_bsg_host_add function to silence
to make htmldocs

Signed-off-by: Marcos Paulo de Souza <marcos.mage@gmail.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>