drivers: adc: stm32: dma fixes

Add a blank define for the case where DMA channels are defined in ADC node
of the dts but STM32_ADC_DMA is not enabled. Otherwise compilation fails.
Also fix the way the DMA channel is configured by using a standard DT
macro, otherwise it doesn't work for dma-v2bis DMA types.

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