dma: mv_xor: Remove all interrupt magic numbers

This commit replaces the current magic numbers in the interrupt handling
with proper macros, which makes more readable and self-documenting.

While here replace the BUG() with a noisy WARN_ON(). There's no reason
to tear down the entire system for an DMA IRQ error.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>