regulator: max8660: Constify regulator_ops

Static struct regulator_ops (except max8660_dcdc_ops) are not modified
so can be made const for code safeness.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>