MIPS: Fix oops in dma_unmap_page on not coherent mips platforms

dma_cache_wback_inv() expects virtual address, but physical was provided
due to translation via plat_dma_addr_to_phys().
If replaced with dma_addr_to_virt(), page fault oops from dma_unmap_page()
is gone on au1550 platform.

Signed-off-by: Jan Nikitenko <jan.nikitenko@gmail.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/dma-default.c

@@ -225,7 +225,7 @@ void dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
 	if (!plat_device_is_coherent(dev) && direction != DMA_TO_DEVICE) {
 		unsigned long addr;
 
-		addr = plat_dma_addr_to_phys(dma_address);
+		addr = dma_addr_to_virt(dma_address);
 		dma_cache_wback_inv(addr, size);
 	}