soc: qcom: smem: fix off-by-one error in qcom_smem_alloc_private()

It's OK if the space for a newly-allocated uncached entry actually
touches the free cached space boundary.  It's only a problem if it
would cross it.

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>