Don't make DEFAULT_DEVICE a constant

I would consider not to have a DEFAULT_DEVICE constant but rather do an actual function call. Even though the default device is at the moment mostly a constant in libdiscid, it might be something which changes on runtime.

I am not entirely sure how OSX handles the devices, but isn't it the case there that the devices change on runtime when the users inserts / removes a disc? At least https://github.com/metabrainz/libdiscid/blob/master/src/disc_darwin.c#L155 suggest this and   Picard has issues with it. Even on other systems that could happen when the user attaches a portable drive, even though dynamic detection of the device is currently not supported by libdiscid on those platforms.