dma: mxs-dma: Cleanup interrupt handler

The DMA interrupt handler uses its controll registers to handle all
available channel interrupts it can find.

This patch changes it to handle only one interrupt by directly mapping
irq number to channel. It also includes a cleanup of the ctrl-register
usage.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>