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

drivers/iio/pressure/bmp280-spi.c

@@ -64,14 +64,14 @@ static int bmp380_regmap_spi_read(void *context, const void *reg,
 	return 0;
 }
 
-static struct regmap_bus bmp280_regmap_bus = {
+static const struct regmap_bus bmp280_regmap_bus = {
 	.write = bmp280_regmap_spi_write,
 	.read = bmp280_regmap_spi_read,
 	.reg_format_endian_default = REGMAP_ENDIAN_BIG,
 	.val_format_endian_default = REGMAP_ENDIAN_BIG,
 };
 
-static struct regmap_bus bmp380_regmap_bus = {
+static const struct regmap_bus bmp380_regmap_bus = {
 	.write = bmp280_regmap_spi_write,
 	.read = bmp380_regmap_spi_read,
 	.read_flag_mask = BIT(7),
@@ -83,7 +83,7 @@ static int bmp280_spi_probe(struct spi_device *spi)
 {
 	const struct spi_device_id *id = spi_get_device_id(spi);
 	const struct bmp280_chip_info *chip_info;
-	struct regmap_bus *bmp_regmap_bus;
+	struct regmap_bus const *bmp_regmap_bus;
 	struct regmap *regmap;
 	int ret;