iio: adc: imx8qxp-adc: propagate regulator_get_voltage error

iio: adc: imx8qxp-adc: propagate regulator_get_voltage error

If the ADC vref regulator returns an error, for example, if CONFIG_REGULATOR
is not set, the error will be used as a reference voltage.

Introduce a guard for negative return values instead of unconditionally
casting it to u32.

Acked-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Martin Larsson <martin.larsson@actia.se>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20220720153136.3502440-1-martin.larsson@actia.se
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>