drivers: dma: stm32: fix dma_stop stream busy handling for HAL override

For DMA channels overridden by HAL DMA, there was no way of
resetting stream busy variable.

Co-authored-by: Maciej Sobkowski <msobkowski@antmicro.com>
Signed-off-by: Dominik Lau <dlau@internships.antmicro.com>