regulator: max8997: Remove unnecessary **rdev from struct max8997_data

Now we are using devm_regulator_register(), so we don't need to allocate *rdev[]
array to store return value of devm_regulator_register. Use a *rdev variable is
enough for checking return status.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>