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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>