spcom: check user space command size

The user space spcomlib provides command buffer
for various commands.
Verify that the command buffer size matches the expected
command struct size.

CRs-Fixed: 1094143
Change-Id: If3ead54bd03368fa9338921e299b2ad8fb078297
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>