drivers/mm7660: correct cast of phydat_t*
drivers/mm7660: correct cast of phydat_t*

The read functions have a mma7660_data_t* argument which is a
struct with 3 int16_t members. Thus we should cast res->val instead of
just res.

aeneby/RIOT,cladmi/RIOT,BytesGalore/RIOT,josephnoir/RIOT,rfuentess/RIOT,smlng/RIOT,mfrey/RIOT,jasonatran/RIOT,jasonatran/RIOT,toonst/RIOT,toonst/RIOT,mtausig/RIOT,aeneby/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,basilfx/RIOT,aeneby/RIOT,mfrey/RIOT,josephnoir/RIOT,smlng/RIOT,x3ro/RIOT,rfuentess/RIOT,miri64/RIOT,aeneby/RIOT,A-Paul/RIOT,toonst/RIOT,kYc0o/RIOT,OTAkeys/RIOT,mfrey/RIOT,yogo1212/RIOT,toonst/RIOT,RIOT-OS/RIOT,ant9000/RIOT,smlng/RIOT,toonst/RIOT,lazytech-org/RIOT,yogo1212/RIOT,rfuentess/RIOT,authmillenon/RIOT,OTAkeys/RIOT,x3ro/RIOT,josephnoir/RIOT,lazytech-org/RIOT,A-Paul/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,basilfx/RIOT,BytesGalore/RIOT,smlng/RIOT,basilfx/RIOT,cladmi/RIOT,kaspar030/RIOT,mfrey/RIOT,OTAkeys/RIOT,x3ro/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,aeneby/RIOT,cladmi/RIOT,x3ro/RIOT,basilfx/RIOT,lazytech-org/RIOT,x3ro/RIOT,ant9000/RIOT,A-Paul/RIOT,authmillenon/RIOT,basilfx/RIOT,yogo1212/RIOT,josephnoir/RIOT,OlegHahm/RIOT,smlng/RIOT,yogo1212/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,lazytech-org/RIOT,rfuentess/RIOT,OTAkeys/RIOT,mtausig/RIOT,rfuentess/RIOT,A-Paul/RIOT,mtausig/RIOT,kaspar030/RIOT,mtausig/RIOT,BytesGalore/RIOT,authmillenon/RIOT,mfrey/RIOT,ant9000/RIOT,A-Paul/RIOT,BytesGalore/RIOT,BytesGalore/RIOT,jasonatran/RIOT,yogo1212/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,mtausig/RIOT,josephnoir/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,authmillenon/RIOT,OlegHahm/RIOT,cladmi/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,cladmi/RIOT,RIOT-OS/RIOT,authmillenon/RIOT