msm: adsprpc: Copy correct size for async job

Copy correct size in invoke2 for async job to avoid
stack out-of-bounds issue.

Change-Id: Ia4ba5acf372ef46ab89b860c7d9ce47f49a9672e
Acked-by: Deepika Singh <dsi@qti.qualcomm.com>
Signed-off-by: Jeya R <jeyr@codeaurora.org>