drivers/mtd_spi_nor: cleanup
drivers/mtd_spi_nor: cleanup

- remove forward declarations
- remove useless do { … } while (0); loops
- insert whitespace

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