sh: Disable get_dma_error_irq for non-SH4 targets.

dma-sh's get_dma_error_irq() is only used by SH4, as the SH3
doesn't have the DMA Error interrupt.

Disable it out for non-SH4 builds.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>