MS5611 error handling improvements

[LorenzMeier]

The MS5611 error handling needs to be improved, as the the following code segment doesn't evaluate correctly due to the mask (identified by Evan Slatyer):

    uint8_t status = val[0] & 0xC0;

    if (status == 2) {
        log("err: stale data");
                perf_count(_comms_errors);
                perf_end(_sample_perf);
        return ret;
    } else if (status == 3) {
        log("err: fault");
                perf_count(_comms_errors);
                perf_end(_sample_perf);
        return ret;                
    }