CC3D - Attempt MPU6000 SPI detection multiple times.

On two CC3D boards tested prior to this fix the mpu6000ReadRegister call
for the MPU6000_WHOAMI would sometimes fail on code boot and sometimes
on warm boot, randomly.  

The reset added in 4db1d58aaf3476aba46c8ffb6af9b364abc2ff70 helps but
didn't fix detection for good.
Merge pull request #186 from cleanflight/cc3d-mpu6000-detection-fix

CC3D - Attempt MPU6000 SPI detection multiple times.