Fix some format string warnings from clang

Building with clang gives some format string warnings as below, fix them
by using the correct format string when printing out the values.

  .../libam7xxx/src/am7xxx.c:804:38: warning:
  format specifies type 'unsigned char' but the argument has type 'int'
  [-Wformat]
                        (*dev)->desc->configuration, current_configuration);
                                                     ^~~~~~~~~~~~~~~~~~~~~
  .../libam7xxx/src/am7xxx.c:65:85: note:
  expanded from macro 'debug'
  #define debug(ctx, ...)   log_message(ctx, AM7XXX_LOG_DEBUG, __func__, 0, __VA_ARGS__)
                                                                            ^
  1 warning generated.

  [...]

  .../libam7xxx/examples/am7xxx-modeswitch.c:75:5:
  warning: format specifies type 'unsigned char' but the argument has type
  'int' [-Wformat]
                                  AM7XXX_STORAGE_CONFIGURATION);
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  .../libam7xxx/examples/am7xxx-modeswitch.c:26:38:
  note: expanded from macro 'AM7XXX_STORAGE_CONFIGURATION'
  #define AM7XXX_STORAGE_CONFIGURATION 1
                                       ^
  .../libam7xxx/examples/am7xxx-modeswitch.c:87:4:
  warning: format specifies type 'unsigned char' but the argument has type
  'int' [-Wformat]
                          AM7XXX_STORAGE_INTERFACE);
                          ^~~~~~~~~~~~~~~~~~~~~~~~
  .../libam7xxx/examples/am7xxx-modeswitch.c:27:38:
  note: expanded from macro 'AM7XXX_STORAGE_INTERFACE'
  #define AM7XXX_STORAGE_INTERFACE     0
                                       ^
  .../libam7xxx/examples/am7xxx-modeswitch.c:104:4:
  warning: format specifies type 'unsigned char' but the argument has type
  'int' [-Wformat]
                          AM7XXX_STORAGE_CONFIGURATION, current_configuration);
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  .../libam7xxx/examples/am7xxx-modeswitch.c:26:38:
  note: expanded from macro 'AM7XXX_STORAGE_CONFIGURATION'
  #define AM7XXX_STORAGE_CONFIGURATION 1
                                       ^
  .../libam7xxx/examples/am7xxx-modeswitch.c:104:34:
  warning: format specifies type 'unsigned char' but the argument has type
  'int' [-Wformat]
                          AM7XXX_STORAGE_CONFIGURATION, current_configuration);
                                                        ^~~~~~~~~~~~~~~~~~~~~
  4 warnings generated.