dma-mapping: properly stub out the DMA API for !CONFIG_HAS_DMA

This avoids link failures in drivers using the DMA API, when they
are compiled for user mode Linux with CONFIG_COMPILE_TEST=y.

Fixes: 356da6d0 ("dma-mapping: bypass indirect calls for dma-direct")
Signed-off-by: Christoph Hellwig <hch@lst.de>