max8998->dev is NULL in current implementation, set it to &pdev->dev.

regulator_register() still return success if max8998->dev is NULL,
but rdev->dev.parent will be set to NULL which is incorrect.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>