dma: pl08x: allow zero slave channels

It might happen that a platform wants to use its DMA engine for
memcpy only, and then we have zero slave channels to initialize,
so allow the slave initialization to return zero.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>