iio: dac: mcp4728: rename err to ret in probe function

To prepare for using a function that returns a non-error value, rename
the variable 'err' to 'ret' in the probe function.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240712-iio-regulator-refactor-round-3-v1-1-835017bae43d@baylibre.com


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