drivers: adc: stm32: driver now agnostic of actual dma name

Previously the STM32 DMA driver was dependent on a very specific
name for the DMA in the DTS. This hidden requirement has caused
a bit of confusion. This commit changes the driver to instead
always use the first DMA listed in the ADC node's dma property.

Should fix: #65387

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>