smcinvoke: populate result from correct returned variable

SCM driver upstream effort has broken how results from trusted side
used to be handled. Since, all 4 return variables (return, res[0-2]
are not available, use correct available variable to fill up result.

Change-Id: Iaf930bb1b70a91b34a14490ab1c2c2794c7c0a48
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>