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