[fix] warning: use of deprecated item: prefer using indexing

Fixing this in just the `src/hal/lpc17xx/pin_pt.rs` first results in:
```
error: mismatched types: expected `core::option::Option<std::collections::hashmap::HashMap<collections::string::String,uint>>` but found an `&`-pointer pattern
```
Fixing the above error, gives this:
```
error: cannot move out of dereference (dereference is implicit, due to indexing)
```