drivers: dma: si32: Improve start and stop logic

As per dma_stop() documentation, the implementation has to not just stop
the transfer, but also to disable the channel. Before this commit, only
the transfer has been stopped.

Same goes for dma_start(). This also allows the enabling of a channel to
be removed from dma_si32_config().

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>