iio: pressure: bmp280: correct meas_time_us calculation

iio: pressure: bmp280: correct meas_time_us calculation

Correction of meas_time_us initialization based on an observation and
partial patch by David Lechner.

The constant part of the measurement time (as described in the
datasheet and implemented in the BM(P/E)2 Sensor API) was apparently
forgotten (it was already correctly applied for the BMP380) and is now
used.

There was also another thinko in bmp280_wait_conv:
data->oversampling_humid can actually have a value of 0 (for an
oversampling_ratio of 1), so it can not be used to detect the presence
of the humidity measurement capability.  Use
data->chip_info->oversampling_humid_avail instead, which is NULL for
chips that cannot measure humidity and therefore must skip that part
of the calculation.

Closes: https://lore.kernel.org/linux-iio/875xgfg0wz.fsf@Gerda.invalid/
Fixes: 26ccfaa9ddaa ("iio: pressure: bmp280: Use sleep and forced mode for oneshot captures")
Suggested-by: David Lechner <dlechner@baylibre.com>
Tested-by: Achim Gratz <Achim.Gratz@Stromeko.DE>
Signed-off-by: Achim Gratz <Achim.Gratz@Stromeko.DE>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>