dma-contiguous: Retrun 'zero' pfn in case of error

When CMA allocator gets error return from the page
allocator framework, except for the -EBUSY case, it
will bail out. Caller depends on the 'pfn' for
confirming allocation successful or not. Return 0
for those error case.

Change-Id: Ica4e04a9f9f18b1a29035ba2bae9deecfd68a5e8
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>