bcm2835-dma: Fix dma_abort for non-40bit channels

The sequence we were doing was not safe.

Clearing CS meant BCM2835_DMA_WAIT_FOR_WRITES was cleared
and so polling BCM2835_DMA_WAITING_FOR_WRITES has no benefit

Broadcom have provided a recommended sequence to abort
a dma lite channel, so switch to that.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>