sh: dma: fix request_irq usage

When calling request_irq with IRQF_SHARED, the dev cookie must be set
(i.e. non-NULL), otherwise the code rejects it immediately with -EINVAL.
So restore the logic here where we'd pass a pointer to the name as a
dummy unique val.

Otherwise, booting up on my LANDISK system would fail with:
	DMAC Address Error0 request_irq fail

This was introduced in commit 7f47c718.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>