iio: adc: ad7266: convert probe to full device-managed

Convert probe functions to device-managed variants, with exception of
the regulator, which required a devm_add_action_or_reset() hook
registration.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Maíra Canal <maira.canal@usp.br>
Link: https://lore.kernel.org/r/20220407115621.10781-1-maira.canal@usp.br


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>