dmaengine: shdma: fix a build failure on platforms with no DMA support

On platforms with no support for the shdma dmaengine driver build is
currently failing with

drivers/built-in.o: In function `sh_mobile_sdhi_probe':
drivers/mmc/host/sh_mobile_sdhi.c:170: undefined reference to`shdma_chan_filter'

Fix the breakage by defining shdma_chan_filter to NULL in such
configurations.
Signed-off-by: NGuennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
[horms+renesas@verge.net.au: Apply change to shdma-base.h instead of sh_dma.h]
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>
Signed-off-by: NOlof Johansson <olof@lixom.net>

include/linux/shdma-base.h

@@ -124,6 +124,10 @@ void shdma_chan_remove(struct shdma_chan *schan);
 int shdma_init(struct device *dev, struct shdma_dev *sdev,
 		    int chan_num);
 void shdma_cleanup(struct shdma_dev *sdev);
+#if IS_ENABLED(CONFIG_SH_DMAE_BASE)
 bool shdma_chan_filter(struct dma_chan *chan, void *arg);
+#else
+#define shdma_chan_filter NULL
+#endif
 
 #endif