soc: qcom: Fix error propagation in scm_qcpe driver

Fix scm_qcpe such that an error returned in X0, will be
propagated back to the calling client.

Change-Id: I8014b3f2f850d8664404c69e77ba36295a28db07
Signed-off-by: Amit Blay <ablay@codeaurora.org>