fix #2741: some devices do not have a magnetic field sensor
fix #2741: some devices do not have a magnetic field sensor

brok85/cgeo,KublaikhanGeek/cgeo,samueltardieu/cgeo,cgeo/cgeo,ThibaultR/cgeo,rsudev/c-geo-opensource,xiaoyanit/cgeo,xiaoyanit/cgeo,marco-dev/c-geo-opensource,schwabe/cgeo,Huertix/cgeo,S-Bartfast/cgeo,KublaikhanGeek/cgeo,marco-dev/c-geo-opensource,auricgoldfinger/cgeo,vishwakulkarni/cgeo,mucek4/cgeo,Huertix/cgeo,vishwakulkarni/cgeo,yummy222/cgeo,Huertix/cgeo,madankb/cgeo,lewurm/cgeo,KublaikhanGeek/cgeo,matej116/cgeo,superspindel/cgeo,lewurm/cgeo,superspindel/cgeo,kumy/cgeo,S-Bartfast/cgeo,SammysHP/cgeo,mucek4/cgeo,samueltardieu/cgeo,rsudev/c-geo-opensource,SammysHP/cgeo,Bananeweizen/cgeo,S-Bartfast/cgeo,madankb/cgeo,schwabe/cgeo,auricgoldfinger/cgeo,tobiasge/cgeo,brok85/cgeo,marco-dev/c-geo-opensource,SammysHP/cgeo,Bananeweizen/cgeo,yummy222/cgeo,ThibaultR/cgeo,cgeo/cgeo,cgeo/cgeo,brok85/cgeo,mucek4/cgeo,tobiasge/cgeo,ThibaultR/cgeo,madankb/cgeo,pstorch/cgeo,schwabe/cgeo,superspindel/cgeo,tobiasge/cgeo,matej116/cgeo,matej116/cgeo,yummy222/cgeo,rsudev/c-geo-opensource,schwabe/cgeo,samueltardieu/cgeo,kumy/cgeo,auricgoldfinger/cgeo,vishwakulkarni/cgeo,kumy/cgeo,Bananeweizen/cgeo,pstorch/cgeo,cgeo/cgeo,xiaoyanit/cgeo,pstorch/cgeo,lewurm/cgeo