dmaengine: shdma: Make dummy shdma_chan_filter() always return false

If CONFIG_SH_DMAE_BASE (which is required for DMA engine support for legacy SH, SH/R-Mobile, and R-Car Gen1, but not for R-Car Gen2) is not enabled, but CONFIG_RCAR_DMAC (for R-Car Gen2 DMA engine support) is, and the DTS doesn't provide a "dmas" property for a device, dma_request_slave_channel_compat() incorrectly succeeds, and returns a DMA channel.

However, when trying to use that DMA channel later, it fails with:

    rcar-dmac e6700000.dma-controller: rcar_dmac_prep_slave_sg: bad parameter: len=1, id=-22

(Fortunately most drivers can handle this failure, and fall back to PIO)

The reason for this is that a NULL legacy filter function is used, which actually means "all channels are OK", not "do not match".
If CONFIG_SH_DMAE_BASE is enabled (like in shmobile_defconfig, which supports other SoCs besides R-Car Gen2), shdma_chan_filter() correctly returns false, as no available channel on R-Car Gen2 matches a shdma-base channel.
If the DTS does provide a "dmas" property, dma_request_slave_channel() succeeds, and legacy filter-based matching is not used.

To fix this, change shdma_chan_filter from being NULL to a dummy function that always returns false, like is done on other platforms.