mfd: mc13xxx-spi: Constify struct regmap_bus

`regmap_mc13xxx_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://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-23-0c8785b1331d@gmail.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/mc13xxx-spi.c

@@ -116,7 +116,7 @@ static int mc13xxx_spi_write(void *context, const void *data, size_t count)
  * single transfer.
  */
 
-static struct regmap_bus regmap_mc13xxx_bus = {
+static const struct regmap_bus regmap_mc13xxx_bus = {
 	.write = mc13xxx_spi_write,
 	.read = mc13xxx_spi_read,
 };