regulator: max8649: Constify struct regmap_config and regulator_ops

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const. Make struct
regulator_ops const as well.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>