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: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NVinod Koul <vinod.koul@intel.com>
Signed-off-by: NFabio Baltieri <fabio.baltieri@linaro.org>