examples: fix 3 compiler warnings

examples: fix 3 compiler warnings

fxload.c:202:17: warning: comparison of integers of different signs: 'int' and
      'unsigned long' [-Wsign-compare]
  ...for (j=0; j<ARRAYSIZE(known_device); j++) {
               ~^~~~~~~~~~~~~~~~~~~~~~~~
fxload.c:222:12: warning: comparison of integers of different signs: 'int' and
      'unsigned long' [-Wsign-compare]
                                        if (j < ARRAYSIZE(known_device)) {
                                            ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~
fxload.c:263:13: warning: comparison of integers of different signs: 'int' and
      'unsigned long' [-Wsign-compare]
        for (i=0; i<ARRAYSIZE(path); i++) {
                  ~^~~~~~~~~~~~~~~~

Signed-off-by: Ludovic Rousseau <ludovic.rousseau@free.fr>