argument: Fix readability-else-after-return

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../argument.cpp:55:5: error: do not use 'else' after 'return' [readability-else-after-return,-warnings-as-errors]
    else
    ^~~~
```

Change-Id: If366f339e5b72c2509509db8b5290f77f96f064b
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed