diag: Fix to check for command length size

Fix to validate payload length size while forwarding
commands to remote processor.

CRs-Fixed: 2004417
Change-Id: Ia047f7895002409e176eeb9e1ddfef69849636ea
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>