AP_InertialSensor: prevent a lockup in MPU6000 driver

thanks to the VRBrain port for noticing this bug.

Failing to get the semaphore is an expected error with the MPU6000, as
we read data both from timer context and mainline code. That means
semaphore conflicts are inevitable. We shouldn't consider them an
error, and shouldn't panic when some arbitrary number of them have
happened since boot.

Instead the wait_for_sample() code checks that we receive new data at
least every 50ms. That is a much safer test.