dma-mapping: Fix build error unused-value

If CONFIG_DMA_DECLARE_COHERENT is not set,
make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- will be failed, like this:

drivers/remoteproc/remoteproc_core.c: In function ‘rproc_rvdev_release’:
./include/linux/dma-map-ops.h:182:42: error: statement with no effect [-Werror=unused-value]
 #define dma_release_coherent_memory(dev) (0)
                                          ^
drivers/remoteproc/remoteproc_core.c:464:2: note: in expansion of macro ‘dma_release_coherent_memory’
  dma_release_coherent_memory(dev);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

The return type of function dma_release_coherent_memory in CONFIG_DMA_DECLARE_COHERENT area is void, so in !CONFIG_DMA_DECLARE_COHERENT area it should neither return any value nor be defined as zero.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: e61c4514

 ("dma-mapping: Add dma_release_coherent_memory to DMA API")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220630123528.251181-1-renzhijie2@huawei.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>