iio: ti-adc128s052: Drop variable vref

iio: ti-adc128s052: Drop variable vref

According to Jonathan, variable reference voltages are very rare. It is
unlikely it is needed, and supporting it makes the code a bit more
complex.

Simplify the driver and drop the variable vref support.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/59106e24332743a7f9eb0b13ad6a2f5595ab485a.1745823530.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>