MPU6050: Stop driver when fifo overflows

The driver should _never_ lose sync since it cannot recover so
stop sending any data instead of phase shifted data.  There should
be code to recover the driver after this.
MPU60X0: Use a labeled PIOS_MPU60X0_INT_STATUS_OVERFLOW flag

Crytic unlabeled constants like 0x10 are bad.  Also there was a
flag that was defined but unused and also didn't match the
datasheet (PIOS_MPU60X0_INT_STATUS_FIFO_FULL) so I removed it.