soc: qcom: smem: Validate heap size against smem area size

Partition size is not available for heap, hence validating
it against smem area size instead of partition size.

CRs-Fixed: 2456912
Change-Id: I20e07c87e00c600fc1adbd9913083b7691dc901c
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>