regulator: hi6421: Fix getting wrong drvdata

Since config.dev = pdev->dev.parent in current code, so
dev_get_drvdata(rdev->dev.parent) call in hi6421_regulator_enable
returns the drvdata of the mfd device rather than the regulator. Fix it.

This was broken while converting to use simplified DT parsing because the
config.dev changed from pdev->dev to pdev->dev.parent for parsing the
parent's of_node.

Fixes: 29dc269a

 ("regulator: hi6421: Convert to use simplified DT parsing")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20210630095959.2411543-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>