stm32/adc_g0: adc async read with dma always uses full resolution

[phycrax]

On STM32G030C8T6 with PA0 connected to VDD:

// This prints 255 as expected
let mut adc = Adc::new(p.ADC1);
adc.set_resolution(BITS8);
dbg!(adc.blocking_read(&mut p.PA0));

// This prints 4095
let mut adc = Adc::new(p.ADC1);
adc.set_resolution(BITS8);
let mut ch0 = p.PA0.degrade_adc();
let mut vals = [0; 1];
adc.read(
    p.DMA1_CH5.reborrow(),
    [
        (&mut ch0, CYCLES160_5),
    ]
    .into_iter(),
    &mut vals,
)
.await;
dbg!(vals[0]);