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: 26ccfaa ("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>

Author: Achim Gratz

drivers/iio/pressure/bmp280-core.c

@@ -1040,13 +1040,16 @@ static int bmp280_wait_conv(struct bmp280_data *data)
 	unsigned int reg, meas_time_us;
 	int ret;
 
-	/* Check if we are using a BME280 device */
-	if (data->oversampling_humid)
-		meas_time_us = BMP280_PRESS_HUMID_MEAS_OFFSET +
-				BIT(data->oversampling_humid) * BMP280_MEAS_DUR;
+	/* Constant part of the measurement time */
+	meas_time_us = BMP280_MEAS_OFFSET;
 
-	else
-		meas_time_us = 0;
+	/*
+	 * Check if we are using a BME280 device,
+	 * Humidity measurement time
+	 */
+	if (data->chip_info->oversampling_humid_avail)
+		meas_time_us += BMP280_PRESS_HUMID_MEAS_OFFSET +
+				BIT(data->oversampling_humid) * BMP280_MEAS_DUR;
 
 	/* Pressure measurement time */
 	meas_time_us += BMP280_PRESS_HUMID_MEAS_OFFSET +