net: dsa: ar9331: constify struct regmap_bus

`ar9331_sw_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: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20240703-net-const-regmap-v1-4-ff4aeceda02c@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/qca/ar9331.c

@@ -1021,7 +1021,7 @@ static const struct regmap_config ar9331_mdio_regmap_config = {
 	.cache_type = REGCACHE_MAPLE,
 };
 
-static struct regmap_bus ar9331_sw_bus = {
+static const struct regmap_bus ar9331_sw_bus = {
 	.reg_format_endian_default = REGMAP_ENDIAN_NATIVE,
 	.val_format_endian_default = REGMAP_ENDIAN_NATIVE,
 	.read = ar9331_mdio_read,