dmaengine: axi-dmac: Return IRQ_NONE if no IRQs are pending

Return IRQ_NONE in the interrupt handler when it is called but no IRQs are
pending. This allows the system to recover in case of an interrupt storm
e.g. due to a wrong interrupt configuration setup.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>