iio: adc: rzg2l_adc: Use read_poll_timeout()

Replace the driver-specific implementation with the read_poll_timeout()
function. This change simplifies the code and improves maintainability by
leveraging the standardized helper.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/20241206111337.726244-7-claudiu.beznea.uj@bp.renesas.com


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