gunyah: gh_rm_drv: Avoid integer overflow for req_payload_size calculation

There could be a potential integer overflow while calculating the
response payload size. Because of this, incorrect memory size could be
copied or allocated. Add a check so that there is no overflow.

Change-Id: I876fbaa704ba51dc4a9e4eb4d3f7f0eaa749e06d
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>