1227: stm32/dma: fix spurious transfer complete interrupts r=Dirbaio a=pattop
DMA interrupts must be acknowledged by writing to the DMA_{L,H}IFCR
register.
Writing to the CR register is unnecessary as the channel (EN bit) is
disabled by hardware on completion of the transfer.
Co-authored-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>