sh: dma: More legacy cpu dma chainsawing.

Attempt to "tidy" up some of the multi IRQ handling and base + IRQ
management. This should keep it limping along without too much hassle,
and no new parts should ever be enabling or using this API anyways.

It doesn't get any closer to lipstick on a pig as this.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>