soc: qcom: smem: check sooner in qcom_smem_set_global_partition()

There's no sense in scanning the partition table again if we know
the global partition has already been discovered.  Check for a
non-null global_partition pointer in qcom_smem_set_global_partition()
immediately.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>