ARM: mach-shmobile: mark DMA slave ID 0 as invalid

This makes it possible to leave DMA slave IDs in the platform data
at default 0 value without hitting DMA channel allocation error paths.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>