iio: pressure: bmp280: Generalize read_*() functions

iio: pressure: bmp280: Generalize read_*() functions

Add the coefficients for the IIO standard units and the IIO value
inside the chip_info structure.

Move the calculations for the IIO unit compatibility from inside the
read_{temp,press,humid}() functions and move them to the general
read_raw() function.

In this way, all the data for the calculation of the value are
located in the chip_info structure of the respective sensor.

Signed-off-by: Vasileios Amoiridis <vassilisamir@gmail.com>
Acked-by: Adam Rizkalla <ajarizzo@gmail.com>
Link: https://patch.msgid.link/20240628171726.124852-2-vassilisamir@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>