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'

IEC 61131-3
===========

    IEC-61131 is a normative document provided by the standards organization IEC
    (International Electrotechnical Commission) and describing a standard for
    implementing programmable controllers.


    The part 3 of this document (commonly named IEC 61131-3) specifies syntax and 
    semantics for programming language for programmable controllers. Beremiz
    implements all the languages described in this document. 


    http://www.iec.eu
        
PLCopen TC6
===========
   
    PLCopen is a vendor- and product-independent worldwide association defining
    international standards for various topics related to control programming.
    For this purpose, PLCopen has 6 technical committees.
   
   
    The goal of the sixth committee (TC6) is to define a standard file format, 
    based on XML, for exchanging programmables controllers programmed using 
    IEC 61131-3 languages. Beremiz uses this file format for saving the PLC
    programs of projects.
   
   
    http://www.plcopen.org