sh: declared coherent memory support V2 fix

This patch fixes the recently introduced declared coherent memory support.
Without this fix a cached memory area is returned by dma_alloc_coherent() -
unless dma_declare_coherent_memory() has setup a separate area.

This patch makes sure an uncached memory area is returned. With this patch
it is now possible to ping through an rtl8139 interface on r2d-plus.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>