iio: adc: ad7091r8: Constify struct regmap_bus

`ad7091r8_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: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240703-iio-cont-regmap_bus-v1-4-34754f355b65@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7091r8.c

@@ -159,7 +159,7 @@ static int ad7091r_regmap_bus_reg_write(void *context, unsigned int reg,
 	return spi_write(spi, &st->tx_buf, 2);
 }
 
-static struct regmap_bus ad7091r8_regmap_bus = {
+static const struct regmap_bus ad7091r8_regmap_bus = {
 	.reg_read = ad7091r_regmap_bus_reg_read,
 	.reg_write = ad7091r_regmap_bus_reg_write,
 	.reg_format_endian_default = REGMAP_ENDIAN_BIG,