iio: pressure: bmp280: Refactorize reading functions

For BMP18x, BMP28x, BME280, BMP38x the reading of the pressure
value requires an update of the t_fine variable which happens
through reading the temperature value.

So all the bmpxxx_read_press() functions of the above sensors
are internally calling the equivalent bmpxxx_read_temp() function
in order to update the t_fine value. By just looking at the code
this functionality is a bit hidden and is not easy to understand
why those channels are not independent.

This commit tries to clear these things a bit by splitting the
bmpxxx_{read/compensate}_{temp/press/humid}() to the following:

i. bmpxxx_read_{temp/press/humid}_adc(): read the raw value from
the sensor.

ii. bmpxx_calc_t_fine(): calculate the t_fine variable.

iii. bmpxxx_get_t_fine(): get the t_fine variable.

iv. bmpxxx_compensate_{temp/press/humid}(): compensate the adc
values and return the calculated value.

v. bmpxxx_read_{temp/press/humid}(): combine calls of the
aforementioned functions to return the requested value.
Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Vasileios Amoiridis <vassilisamir@gmail.com>
Link: https://lore.kernel.org/r/20240512230524.53990-2-vassilisamir@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/pressure/bmp280.h

@@ -397,12 +397,6 @@ struct bmp280_data {
 	 */
 	int sampling_freq;
 
-	/*
-	 * Carryover value from temperature conversion, used in pressure
-	 * calculation.
-	 */
-	s32 t_fine;
-
 	/*
 	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.