msm: ADSPRPC: Validate buffer size

Ensure that the size of buffer being allocated is not
negative to avoid allocating a smaller buffer and hence
causing buffer overflow conditions.

Change-Id: I150c379471564731771e2bd33d804fd26c793e25
Acked-by: Bharath Kumar <bkumar@qti.qualcomm.com>
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>