iio: adc: ti-adc108s102: Use devm managed functions for all of probe()

Simplifies error handling and lets us drop remove() entirely.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Bogdan Pricop <bogdan.pricop@emutex.com>
Reviewed-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210516172520.1398835-8-jic23@kernel.org