MIPS: don't select DMA_MAYBE_COHERENT from DMA_PERDEV_COHERENT

While both option select a form of conditional dma coherence they don't
actually share any code in the implementation, so untangle them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 3551199..0b25180 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1111,7 +1111,7 @@
 
 config DMA_PERDEV_COHERENT
 	bool
-	select DMA_MAYBE_COHERENT
+	select DMA_NONCOHERENT
 
 config DMA_NONCOHERENT
 	bool