Fix pianoIsInserted always returning false

While looking for piano example code, I noticed [this oddity](https://github.com/CamdenFlannagan/NDS-Audio-Project-1/blob/master/main.c#L255): ```c // for whatever reason, the piano stuff won't work without checking for the guitar grip first // oh well. guitarGripIsInserted(); ``` So I put some logging in NitrousTracker (where piano support is currently broken) and found that `pianoIsInserted()` always returns false, _unless_ you call `guitarGripIsInserted()` first. I looked at the [guitar grip](https://github.com/blocksds/libnds/blob/master/source/arm9/peripherals/guitarGrip.c#L14-L25) module, and saw it not only grants ARM9 access to the GBA cartridge bus but also sets some timing flags which are seemingly important. I copied this into the piano module, and now `pianoIsInserted()` is working as expected.