scsi_transport_iscsi: Unexport iscsi_is_flashnode_conn_dev()

The output of "git grep -nHw iscsi_is_flashnode_conn_dev" shows that
this function is only called from inside source file
drivers/scsi/scsi_transport_iscsi.c. Hence unexport this function.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>