Improve fix wrong code generation if EN/ENO are used in FBD/LD/SFC

Commit c9915bc6 "Fix wrong code generation if EN/ENO are used in
FBD/LD/SFC" caused following backtrace in some cases:

File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 476, in GenerateProgram
self.GeneratePouProgram(pou_name)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 258, in GeneratePouProgram
program = pou_program.GenerateProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1713, in GenerateProgram
self.ComputeProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 997, in ComputeProgram
eno_var = self.GetUsedEno(body, connections)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 929, in GetUsedEno
for invar in blk.inputVariables.getvariable():
File "/home/developer/WorkData/PLC/beremiz/beremiz/xmlclass/xmlclass.py", line 1363, in getattrMethod
return DefaultElementClass.__getattribute__(self, name)
<type 'exceptions.AttributeError'>: 'inOutVariable' object has no attribute 'inputVariables'

#Encoding of BACnet Object Types;;;
#Code;Object Type;;
0;Analog Input;;
1;Analog Output;;
2;Analog Value;;
3;Binary Input ;;
4;Binary Output;;
5;Binary Value;;
6;Calendar;;
7;Command;;
8;Device;;
9;Event-Enrollment;;
10;File;;
11;Group;;
12;Loop;;
13;Multistate Input;;
14;Multistate Output
15;Notification Class
16;Program
17;Schedule
18;Averaging
19;Multistate Value
20;Trend Log
21;Life Safety Point
22;Life Safety Zone
23;Accumulator
24;Pulse Converter