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: Guennadi 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: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Olof Johansson <olof@lixom.net>