dmaengine: set_dma40: ignore spurious interrupts

Some DMA channels may be used by other cores in the SoC.  This patch
modifies the dma interrupt handler to ignore interrupts from unknown
channels.

Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>