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.

Change-Id: I0d5a66a25c4483bf0f219cec1d7009239518f27a
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>