iio: adc: ad7476: Simplify scale handling

iio: adc: ad7476: Simplify scale handling

The ad7476 driver supports variants with different amount of supply
regulators. On some variants there is only VCC, which is used as a
reference voltage. Others have separate VREF regulator, and some rely on
internal VREF. Some have both internal VREF and option to connect
external one.

The ad7476 driver reads the regulator voltage only when the user asks to
get the scale. This means the driver needs to do some dancing while
picking the correct reference regulator (or internal reference), and
store it for the later use.

According to the discussion [1] variable voltage references are rare,
making it hard to justify the added complexity for supporting those.

Drop the support for the variable voltage references and simplify things
by using the managed regulator get and enable interfaces.

Link: https://lore.kernel.org/linux-iio/20250331122247.05c6b09d@jic23-huawei/
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: $URL #1
Link: https://patch.msgid.link/4a497cc5ac0f6506c1c94d10849e0ae1cacf1191.1754901948.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>