drivers: dma: stm32: fix build for STM32F0 MCUs

The STM32F0 MCUs (except STM32F030XC) don't have a DMA channel selection
register (DMA_CSELR). This patch fixes the build of the dma_stm32 driver
for them.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>