iio: accel: bma400: Constify struct regmap_bus

`bma400_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>
Link: https://patch.msgid.link/20240703-iio-cont-regmap_bus-v1-2-34754f355b65@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/bma400_spi.c

@@ -53,7 +53,7 @@ static int bma400_regmap_spi_write(void *context, const void *data,
 	return spi_write(spi, data, count);
 }
 
-static struct regmap_bus bma400_regmap_bus = {
+static const struct regmap_bus bma400_regmap_bus = {
 	.read = bma400_regmap_spi_read,
 	.write = bma400_regmap_spi_write,
 	.read_flag_mask = BIT(7),