iio: adc: ad7606: simplify using devm_regulator_get_enable()

iio: adc: ad7606: simplify using devm_regulator_get_enable()

Drop open-coded pattern: 'devm_regulator_get(), regulator_enable(),
add_action_or_reset(regulator_disable)' and use the
devm_regulator_get_enable() and drop the pointer to the regulator.
This simplifies code and makes it less tempting to add manual control
for the regulator which is also controlled by devm.

Whilst here also switch to dev_err_probe() to provide more information
if a deferred probe occurs.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/521c52f5a9bdc2db04d5775b36df4b233ae338da.1660934107.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>