Sparky: fix compilation for mpu9150/mpu6050 option

Previously it didn't compile properly without both options
enabled.
Sparky: disable MPU6050 option for now

Seeing some issues autoprobing it seems like so the mag
was not running properly.
PIOS_MPU9150: read the status registers before starting mag

Now resets to normal mode and reads all status registers, to
ensure starting from a known good configuration
Sparky: refactor #ifdef based on review feedback
Merge pull request #597 from peabody124/sparky_mag_fix

Sparky mag fix