iio:adc:ad7476: Fix remove handling

This driver was in an odd half way state between devm based cleanup
and manual cleanup (most of which was missing).
I would guess something went wrong with a rebase or similar.
Anyhow, this basically finishes the job as a precursor to improving
the regulator handling.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Fixes: 4bb2b8f9 ("iio: adc: ad7476: implement devm_add_action_or_reset")
Cc: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210401171759.318140-2-jic23@kernel.org