iio: adc: ti-adc161s626: Use devm managed functions for all of probe.

Simplifies error handling and allows us to drop remove entirely.

The regulator handling in this driver was unusual as it would try to
acquire the regulator, but if that failed with an error would continue.

We should get a stub regulator if one isn't provided in DT and an error
could indicate an actual problem preventing the device being powered
(perhaps a need to defer). So this handling is cleaned up (arguably
that might be a fix but given no one has run into it, I haven't broken
it out separately.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210516172520.1398835-9-jic23@kernel.org