drivers/dfplayer: fix error handling
drivers/dfplayer: fix error handling

The error code was previously overwritten :-/ this clearly is not how it is
intended to work.

authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT