regmap: constify regmap_bus structures

These regmap_bus structures are only passed as the second argument to
__devm_regmap_init or __regmap_init, both of which are const, so the
regmap_bus structures can be const too.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>