MPU9250 crashes.. Here's the fix :)

[MarcGroef]

Hi,

During testing, I found that my esp32 was crashing when using the MPU9250 from your library.
I found the issue to be that in F_MPU9250.cpp, the function int MPU9250::initMagnetometer() doesn't return an int.

Cheers!

[LiquidCGS]

Oh wow I didn't even notice that, I'll update that soonish