iio: dac: ltc2688: Constify struct regmap_bus

`ltc2688_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>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240703-iio-cont-regmap_bus-v1-6-34754f355b65@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ltc2688.c

@@ -918,7 +918,7 @@ static bool ltc2688_reg_writable(struct device *dev, unsigned int reg)
 	return false;
 }
 
-static struct regmap_bus ltc2688_regmap_bus = {
+static const struct regmap_bus ltc2688_regmap_bus = {
 	.read = ltc2688_spi_read,
 	.write = ltc2688_spi_write,
 	.read_flag_mask = LTC2688_READ_OPERATION,