regmap: Constify static regmap_bus structs

The only usage of these is to pass their address to __regmap_init() or
__devm_regmap_init(), both which takes pointers to const struct
regmap_bus. Make them const to allow the compiler to put them in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20220330214110.36337-1-rikard.falkeborn@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>