AP_Compass: Specify the names of the supported devices

[muramura]

This driver supports not only the AK09916, but also the AK09915 and AK09918. I think it would be better to explicitly list them.

https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Compass/AP_Compass_AK09916.cpp#L340

[peterbarker]

Ping @muramura - changes requested

[peterbarker]

I've fixed the commit list and marked for merging.

[rmackay9]

I suspect there's a decoding script somewhere that should theoretically also be updated..