dma-coherent: fix dma_declare_coherent_memory() logic error

A recent change interprets the return code of dma_init_coherent_memory
as an error value, but it is instead a boolean, where 'true' indicates
success. This leads causes the caller to always do the wrong thing,
and also triggers a compile-time warning about it:

drivers/base/dma-coherent.c: In function 'dma_declare_coherent_memory':
drivers/base/dma-coherent.c:99:15: error: 'mem' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I ended up changing the code a little more, to give use the usual
error handling, as this seemed the best way to fix up the warning
and make the code look reasonable at the same time.

Fixes: 2436bdcd

 ("dma-coherent: remove the DMA_MEMORY_MAP and DMA_MEMORY_IO flags")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>