iio: chemical: bme680: Constify struct regmap_bus

`bme680_regmap_bus` is not modified and can be declared as const to
move its 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-v1-5-34754f355b65@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/chemical/bme680_spi.c

@@ -100,7 +100,7 @@ static int bme680_regmap_spi_read(void *context, const void *reg,
 	return spi_write_then_read(spi, &addr, 1, val, val_size);
 }
 
-static struct regmap_bus bme680_regmap_bus = {
+static const struct regmap_bus bme680_regmap_bus = {
 	.write = bme680_regmap_spi_write,
 	.read = bme680_regmap_spi_read,
 	.reg_format_endian_default = REGMAP_ENDIAN_BIG,