iio: pressure: bmp280: Constify struct regmap_bus

`bmp280_regmap_bus` and `bmp380_regmap_bus` are conditionally assigned
to `bmp_regmap_bus`, which is only used to pass the struct as a
read-only member.

Add the const modifier to the structs and the pointer to move the data
to a read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Tested-By: Vasileios Amoiridis <vassilisamir@gmail.com>
Link: https://patch.msgid.link/20240703-iio-cont-regmap_bus-v1-10-34754f355b65@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>