clippy: fix unwrap_or with call to new()

```
src/config.rs:119:20: 119:97 warning: use of `unwrap_or` followed by a call to `new`, #[warn(or_fun_call)] on by default
src/config.rs:119             names: d.read_struct_field("names", 0, Decodable::decode).unwrap_or(BTreeSet::new()),
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/config.rs:119:20: 119:97 help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#or_fun_call
src/config.rs:119:20: 119:97 help: try this
src/config.rs:                names: d.read_struct_field("names", 0, Decodable::decode).unwrap_or_default(),
```

Note that in order to call `unwrap_or_default` we need to convert
the `Result` type to `Option` via `ok()`.

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

src/config.rs

@@ -116,7 +116,7 @@ impl Decodable for PinConfig {
                             }
                         })
                         .unwrap_or(sysfs_gpio::Direction::In), // default: In
-            names: d.read_struct_field("names", 0, Decodable::decode).unwrap_or(BTreeSet::new()),
+            names: d.read_struct_field("names", 0, Decodable::decode).ok().unwrap_or_default(),
             export: d.read_struct_field("export", 0, Decodable::decode).unwrap_or(true),
             active_low: d.read_struct_field("active_low", 0, Decodable::decode).unwrap_or(false),
         })