Do not fail if mdlStart() cannot open /dev/uio0

Do not fail if mdlStart() cannot open /dev/uio0

Such failing is inconvenient because mdlStart() is called even during
code generation [1]. If one tries to generate the code on a computer
without MF624 hardware, it fails.

This is solved that we ignore error in mdlStart(). When initialization
fails, we simply return from mdlStart() without reporing an error to the
user. The status of initialization is checked in every call to mdlOutputs()
and there the failres are already reported to the user.

[1] http://www.mathworks.com/matlabcentral/newsreader/view_thread/32986