regulator: max77686: Consistently index opmode array by rdev id

Mixed indexes were used for array of opmodes in max77686_data structure:
id of regulator and index of regulator_desc array.

These indexes are exactly the same but the mixture may confuse. Use
consistently the id of regulator.

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