test: sysfs: Fix readability-identifier-naming

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/sysfs.cpp:29:25: error: invalid case style for variable 'MAX_BRIGHTNESS_VAL' [readability-identifier-naming,-warnings-as-errors]
constexpr unsigned long MAX_BRIGHTNESS_VAL = 128;
                        ^~~~~~~~~~~~~~~~~~
                        maxBrightnessVal
```

Change-Id: I8383e1476f31ff2c488b18a97d4a32294327a9c2
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed