regulator: max8997: Convert ot use devm_kzalloc

Also simplify the error handling to start unwind from the place
regulator_register fails.

No need to check rdev[i] is NULL or not before calling regulator_unregister.
regulator_unregister is safe if rdev is NULL,

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>