drivers: adc: stm32: configure data size for stm32n6

On STM32N6, the register holding the data is 32 bits and DMA must operate
in word transfer to work properly. So we change the type of the buffer in
which we store the ADC data from uint16_t to uint32_t for N6.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>