mtd: allow to use non-pagewise functions
mtd: allow to use non-pagewise functions

If the underlying driver implements 'plain' read/write/erase
fucntions, don't convert them to pagewise functions and back.

Just use the old direct functions.

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