Wrong formula for gas_wait causing huge sampling delays

[antoine-sac]

A typo in the code makes the gas sampling take over 4 seconds in most cases.

As per datasheet, _calc_gas_wait(100) should be 0x59 but we always get 0xff instead -- I suggest adding a test with this known-good value.

The code currently reads

durval: UINT8 = 0xFF  # Max duration
if dur < 0xFC0:
    return durval

but it should be

durval: UINT8 = 0xFF  # Max duration
if dur >= 0xFC0:
    return durval

cf https://github.com/boschsensortec/BME68x-Sensor-API/blob/master/bme68x.c#L1181C13-L1181C13

That's probably the root cause of #65 and does not help #66 either (because we block waiting for sleep while gas wait is active).