cma: Return 0 on error path

If a free CMA region cannot be found because every one is busy,
an error needs to be propegated up by returning a zero pfn.
Ensure the pfn is actually zero when returning an error.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

Conflicts:
	drivers/base/dma-contiguous.c