drivers/mag3110: correct cast of phydat_t*
drivers/mag3110: correct cast of phydat_t*

The read functions have a mag3110_data_t* argument which is a
struct with 3 int16_t members. Thus we should cast res->val instead of
just res.

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