regmap-i2c: constify regmap_bus structures

regmap_bus structures are not changed anywhere. Mark them const.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/85e4141348db00ecf1f2bc5c2ff6ba3de75e8ff4.1578134920.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Mark Brown <broonie@kernel.org>