controller: Fix readability-identifier-naming

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../controller.cpp:27:13: error: invalid case style for function 'ExitWithError' [readability-identifier-naming,-warnings-as-errors]
static void ExitWithError(const char* err, char** argv)
            ^~~~~~~~~~~~~
            exitWithError
```

Change-Id: I9fc779fcec82c00cb2168e07703a51548990f066
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/controller.cpp b/controller.cpp
index 8219ef3..cce3f10 100644
--- a/controller.cpp
+++ b/controller.cpp
@@ -24,7 +24,7 @@
 #include <iostream>
 #include <string>
 
-static void ExitWithError(const char* err, char** argv)
+static void exitWithError(const char* err, char** argv)
 {
     phosphor::led::ArgumentParser::usage(argv);
     std::cerr << std::endl;
@@ -93,7 +93,7 @@
     auto path = std::move((options)["path"]);
     if (path.empty())
     {
-        ExitWithError("Path not specified.", argv);
+        exitWithError("Path not specified.", argv);
     }
 
     // If the LED has a hyphen in the name like: "one-two", then it gets passed